Channel-switched tunable laser for DWDM communications

ABSTRACT

Laser source including materials with negative index of refraction dependence on temperature and with temperature independent coincidence between cavity modes and a set of specified frequencies such as DWDM channels in telecommunications applications. The free spectral range may be adjusted to equal a rational fraction of the specified frequency interval. The operating frequency may be defined by a frequency selective feedback element that is thermo-optically tuned by the application of heat from an actuator without substantially tuning the cavity modes. The operating frequency may be induced to hop digitally between the specified frequencies. In a particular embodiment, semiconductor amplifier and polymer waveguide segments form a linear resonator with a thermo-optically tuned grating reflector. In a further embodiment, an amplifier and two waveguides from a tunable grating assisted coupler form a ring resonator. Tuning may also be accomplished by means of applying an electric field across a liquid crystal portion of the waveguide structure within the grating.

TECHNICAL FIELD

This invention relates to laser devices that produce optical energy oftightly controlled optical frequency, particularly for use intelecommunications applications. More particularly, the inventionrelates to devices that produce a specified optical frequencyindependent of thermal variations, while possessing the ability to betuned or switched among alternative optical frequencies by thermal,electric field, or other control means without modifying the specifiedfrequencies.

BACKGROUND ART

The growth of demand for subscriber bandwidth has led to great pressureto expand the capacity of the telecommunications networks. Densewavelength division multiplexing (DWDM) allows high bandwidth use ofexisting fiber, but low-cost cost components are required to enableprovision of high bandwidth to a broad range of customers. Keycomponents include the source, the detector, and routing components, butthese components should preferably be addressable to any of thefrequency channels. These channels are currently defined by the ITU asν_(n)=ν₀±n dν, where ν₀ is the central optical frequency 193.1 THz anddν is the specified frequency channel spacing that may equal a multipleof 100 GHz or 50 GHz. Systems have also been demonstrated based on otherfixed spacings, and based on nonuniform frequency spacings.

Semiconductor lasers with built-in gratings such as DFB and DBR lasersare currently used to produce the frequency-specific lasers needed totransmit over optical fibers. However, current fabrication techniques donot allow high yield production of a given frequency channel because ofindex of refraction variations in the InP-based materials. Becausesilica, polymer, and other optical materials offer greater stability ofindex of refraction, many types of hybrid lasers have been tested inwhich a semiconductor gain medium is combined with a grating fabricatedin another material. Single frequency hybrid waveguide lasers have beendemonstrated with semiconductor waveguide amplifiers to obtain thebenefits of frequency selectivity and tunability. See for example * J.M. Hammer et al., Appl. Phys. Lett. 47 183, (1985), who used a gratingin an external planar waveguide, by * E. Brinkmeyer et al., Elect. Lett22 134 (1986) and * E. I. Gordon, U.S. Pat. No. 4,786,132, Dec. 22, 1988and * R. C. Alferness, U.S. Pat. No. 4,955,028, Sep. 4, 1990, who used agrating in a fiber waveguide, by * D. M. Bird et al., Elect. Lett. 271116 (1991) who used a UV-induced grating, by * W. Morey, U.S. Pat. No.5,042,898, Aug. 27, 1991 who used a fiber grating with thermallycompensated package, by * P. A. Morton et al., Appl. Phys. Lett. 64 2634(1994) who used a chirped grating, by * D. A. G. Deacon, U.S. Pat. No.5,504,772, Apr. 2, 1996, who used multiple gratings with opticalswitches, by * J. N. Chwalek, U.S. Pat. No. 5,418,802, May 23, 1995, whoused an electro-optic waveguide grating, by * R. J. Campbell et al.,Elect. Lett. 32 119 (1996) who used an angled semiconductor diodewaveguide, by * T. Tanaka et al, Elect. Lett. 32 1202 (1996) who usedflip-chip bonding, and by * J-M. Verdiell, U.S. Pat. No. 5,870,417, Feb.9, 1999, who adjust for single mode operation. Single frequency hybridwaveguide lasers have also been demonstrated with fiber waveguideamplifiers. See * D. Huber, U.S. Pat. No. 5,134,620, Jul. 28, 1992 and *F. Leonberger, U.S. Pat. No. 5,317,576, May 31, 1994.

Many robust thermo-optic materials are available today including glassand polymer materials systems that can also be used in fabricatingwaveguide optical components. See * M. Haruna et al., IEE Proceedings131H 322 (1984), and * N. B. J. Diemeer, et al., J. Light. Technology,7, 449-453 (1989). Recently, thermally tunable gratings have beenfabricated in polymer waveguides and resonators. See * L. Eldada et al.,Proceedings of the Optical Fiber Communications Conference, OpticalSociety of America, p. 98 (1999), and * N. Bouadma, U.S. Pat. No.5,732,102, Mar. 24, 1998.

Thermal compensation of laser resonators is a requirement in componentsthat must operate robustly within the narrow absolute frequency bands ofthe DWDM specifications. Thermally compensated resonators have has beenshown using polymer materials. See * K. Tada et al., Optical and QuantumElectronics 16, 463 (1984). Thermally compensated packages for fibergrating based devices have also been shown. See * W. Morey, U.S. Pat.No. 5,042,898, Aug. 27, 1991, * G. W. Yoffe et al, Appl. Opt. 34 6859(1995), and * J-M. Verdiell, U.S. Pat. No. 5,870,417, Feb. 9, 1999.Thermally compensated waveguides using mixed silica-polymer materialshave also been shown to produce temperature independent characteristics.See * Y. Kokubun et al., IEEE Photon. Techn. Lett. 5 1297 (1993), and *D. Bosc, U.S. Pat. No. 5,857,039, Jan. 5, 1999. Silica-polymerwaveguides have also been used for interconnecting laser devices. See *K. Furuya U.S. Pat. No. 4,582,390, Apr. 15, 1986.

The grating assisted coupler is a useful device for frequency control.Grating assisted couplers as described in * R. C. Alferness, U.S. Pat.No. 4,737,007, Apr. 12, 1988, are known in many configurations includingwith mode lockers, amplifiers, modulators, and switches. See * A. S.Kewitsch, U.S. Pat. No. 5,875,272, Feb. 23, 1999. Grating assistedcouplers have been used in resonators including lasers, mode lockers,etalons, add-drop filters, frequency doublers, etc. See for example * E.Snitzer, U.S. Pat. No. 5,459,801, Jan. 19, 1994, and * D. A. G. Deacon,U.S. Pat. No. 5,581,642, Dec. 3, 1996.

What is needed is a laser that operates robustly at a frequencyspecified for DWDM systems, with the operating frequency independent ofenvironmental variations such as temperature and humidity. Ideally, thislaser should also be tunable among many or all of the DWDM channels, andit should be inexpensive and easy to produce and test.

SUMMARY OF THE INVENTION

According to the invention, an amplifier device is combined with amaterial with negative index of refraction dependence on temperature toproduce a laser device with cavity length and index of refractioncontrol to accomplish temperature independent coincidence between cavitymodes and a set of specified frequencies such as the DWDM opticalchannels in telecommunications applications. The free spectral range maybe adjusted to equal a rational fraction of a specified frequencyinterval. The operating frequency may be defined by a frequencyselective feedback element that is thermo-optically tuned by theapplication of heat from an actuator without substantially tuning thecavity modes. The operating frequency may be unique and it may beinduced to hop digitally between the specified frequencies. In aparticular embodiment, semiconductor amplifier and polymer waveguidesegments form a linear resonator with a thermo-optically tuned gratingreflector. In a further embodiment, an amplifier and two waveguides froma tunable grating assisted coupler form a ring resonator. Tuning mayalso be accomplished by means of applying an electric field across aliquid crystal portion of the waveguide structure within the grating.Methods are described of bringing the free spectral range of the cavitywithin tolerance, including intracavity methods of ablating material,depositing material, and exposing material to radiation.

The advantages of the invention include the fact that it provides arobust, athermal set of operating frequencies tied to a specified set ofoptical frequency channels. Digital tuning may be provided among thesechannels by thermal or other means without substantially modifying thespecified frequency channels so that error-free channel selection isenabled among the provided channels. No wavelockers or other means areneeded to specify the channels of operation. A simple method of channelselection is available, and direct modulation of the amplifier medium isavailable for data transmission, avoiding the need for a modulator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an embodiment of a hybrid grating stabilized laser chip.

FIG. 2 is a packaged embodiment in cutaway view of a thermallystabilized integrated optic chip.

FIG. 3 is an embodiment of a laser chip coupled to a tapered waveguide.

FIG. 4 is an illustration of the z-variation of the effective modeindices of two coupled waveguides in a tapered waveguide chip.

FIG. 5 is a top-view schematic diagram of a curved laser waveguide arraywith an angled interface to a tapered waveguide array.

FIG. 6 is an embodiment of a coupling region between a waveguide and aV-groove.

FIG. 7A is an embodiment of a waveguide-end lens.

FIG. 7B is an alternative embodiment of a waveguide-end lens.

FIG. 8 is a segmented embodiment of a tapered waveguide coupler.

FIG. 9 is a codirectional grating assisted coupler embodiment of a ringhybrid laser chip.

FIG. 10 is a reflective grating assisted coupler embodiment of a ringresonator hybrid laser chip.

FIG. 11A is a lateral cross section of a waveguide embodiment withsingle upper cladding.

FIG. 11B is a lateral cross section of a waveguide embodiment withdouble upper cladding.

FIG. 12A shows a hybrid grating stabilized laser embodiment seen in alongitudinal cross section being illuminated for adjusting a frequencyof operation.

FIG. 12B shows a hybrid grating stabilized laser embodiment seen in alongitudinal cross section having material removed for adjusting afrequency of operation.

FIG. 12C shows a hybrid grating stabilized laser embodiment seen in alongitudinal cross section having material being deposited for adjustinga frequency of operation.

FIG. 13 is an embodiment of a tunable frequency-adjusted ring resonatorcross connect device.

FIGS. 14A, 14B, and 14C show lateral cross sections of three stages offabrication of a waveguide pair embodiment with differing waveguidethermal coefficients.

FIG. 15 is a lateral cross section of an alternative waveguide pairembodiment with differing waveguide thermal coefficients.

FIG. 16 is a flow chart of a method for adjusting the free spectralrange of a resonator.

FIG. 17 is a dual grating embodiment of a tunable frequency-selectivecross connect device.

FIG. 18 is an illustration of a vernier method of frequency tuning dualgrating resonant devices.

DESCRIPTION OF PREFERRED EMBODIMENTS

Thermo-optic materials are both convenient and robust, and may beincorporated into optical devices that actuate or tune upon theactuation of a heater. However, for applications such as communications,thermally insensitive devices are required for reliability. Thisinvention describes a unique way of including a highly thermallysensitive tuning component into a device that is, in key aspects of itsoperation, insensitive to temperature. In one embodiment of theinvention, a frequency tunable laser is described that emits light whichmay be channel-switched among a set of prepared frequencies that areindependent of temperature. In another embodiment of the invention, afrequency tunable cross-connect device is described that cross-connectsa frequency which may be channel-switched among a set of preparedfrequencies that are independent of temperature.

Tunable Laser Operation

As is known in the art, laser tuning involves two factors that aretypically interdependent: a) tuning a frequency selective element suchas a grating within whose interaction band operation will occur; and b)tuning the longitudinal modes of the laser that determine the exactoptical frequency that will lase within the band of the frequencyselective element. In the DBR (distributed Bragg reflector) laser, forexample, changing the chip temperature shifts both grating and modefrequencies, typically at different rates, leading to mode hoppingbehavior.

In this invention, means are described for making the cavity freespectral range (FSR) independent of temperature for high reliability.Further, the cavity round trip optical length may be adjusted accordingto methods described herein to adjust the FSR to equal a specifiedfrequency channel spacing. The cavity is thereby prepared to lase at thedesired optical frequencies. If the cavity is designed to be athermal,it may also provide an absolute frequency reference to thecommunications channel frequencies independent of drifts inenvironmental temperature, and no external reference devices such as awavelocker need be provided. The free spectral range of a cavity isgiven by

FSR=c/(nL)_(eff)  (1)

where the round trip optical length (nL)_(eff) is

(nL)_(eff)=Σ_(i)(n_(i eff) L_(i))  (2)

and where n_(i eff) is the effective index of refraction of the mode inthe i^(th) segment traversed by the light in the cavity, L_(i) is thephysical length of the i^(th) segment, and the sum is over all the pathsegments traversed in a round trip of the cavity. The effective index ofan optical mode in a waveguide is the index of refraction of anequivalent uniform medium that would give a plane wave the samepropagation constant

β=2πn_(eff)/λ  (3)

or wavenumber (where the electric field of the mode varies as E ∝e^(iβz)along the direction of propagation).

In an aspect of the present invention, the free spectral range of thecavity may be designed to equal a predetermined DWDM channel spacing setby a system integrator or by the ITU (International TelecommunicationsUnion). In the preferred embodiment, the FSR is 50 GHz so that theeffective round trip optical length of the cavity is 6000 microns. Withthe linear cavity shown in FIG. 1 and a preferred InP semiconductorlaser chip length of 400 microns at an index of about 3.3, the opticallength of the waveguide path between the edge of the laser chip and thestart of the grating is about 1680 microns. If the effective index ofthe waveguide is about 1.45, its physical length is about 1160 microns,ignoring the small butt-coupling gap between the two waveguides.

In a variation of the preferred embodiment, the free spectral range ofthe cavity is designed to equal a rational fraction of a desiredcommunications channel spacing

FSR=(communications channel spacing)·n/m  (4)

where n and m are integers. If the desired channel spacing is 50 GHz,for example, FSR=50·n/m GHZ. This may be useful if n/m<1 to increase thephysical length of the resonator by a factor of m/n, simplifying designand fabrication issues. In one example, if n=1 and m=2, the rationalfraction is ½ and the desired cavity round trip optical path becomes12000 microns, allowing more than double the space in the resonator forthermal compensation material, taper, etc. In this situation, thegrating tuning requirements are still the same, but the laser frequencywill hop in 25 GHZ increments if the grating is tuned continuously, withevery other hop bringing the optical frequency to a desiredcommunications channel. (Some communications systems can use 25 GHzchannel spacing, in which case m=1 for this cavity length.) Or it may beuseful if n>1 to interleave successively addressed frequency channels.In another example, if n=2 and m=1, the rational fraction is 2 and thelaser will hop successively to every second communications channel,which might be useful for interleaving two devices in the frequencydomain, or for increasing device stability against perturbations,ageing, and drifts. These approaches may also be combined, as in anotherexample, with n=2 and m=3 where the round trip optical path becomes 9000microns, and the laser frequency during operation will hop first to+33.3 GHz, then to +66.6 GHz, both frequencies in-between communicationschannels, and then to +100 GHz, a communications frequency two 50 GHzintervals away from the starting channel.

Athermal Cavity

In another aspect of the invention, a region of thermo-optical polymermay be incorporated within the laser resonator where the negativethermo-optic coefficient is exploited to produce an athermal freespectral range. For the FSR to be athermal or independent of devicetemperature, the device parameters may be chosen to satisfy the relation

d (nL)_(eff)/dT=Σ_(i)(dn_(i eff)/dT·L_(i)+n_(i eff)·dL_(i)/dT)=0  (5)

within a tolerance, where the sum is taken over all the differentlongitudinal segments of the cavity along the optical path. See K. Tadaet al., Optical and Quantum Electronics 16,463 (1984). Since thesequantities are all positive in the common non-polymeric materialsincluding silica, silicon, InP, GaAs, glass, lithium niobate, lithiumtantalate, etc., a hybrid or multiple material approach is needed. Theconventional approach to achieving temperature compensation has been tomake a large negative dL/dT in one of the lengths L_(i) in the abovesummation (usually a path length in air) equal to the difference betweentwo large lengths (usually overhanging members in a supportingstructure, one pointing away from the cavity and the other pointingtowards the cavity). By selecting an inward-pointing member to have alarger coefficient of thermal expansion than the correspondingoutward-pointing member, the support structure can be arranged to reducethe path length L_(i) as the temperature is increased. See for exampleW. Morey, U.S. Pat. No. 5,042,898, Aug. 27, 1991, “Incorporated Braggfilter temperature compensated optical waveguide device” and J M.Verdiell, U.S. Pat. No. 5,870,417, Feb. 9, 1999, “Thermal compensatorsfor waveguide DBR laser sources”.

In the preferred embodiment of the present invention, a polymer materialis used to provide the negative thermo-optic coefficient in the claddingof the waveguide, and the waveguide design is adjusted for a negativenet change in index with temperature. The length of the polymerintracavity segment may be adjusted until equation (5) is essentiallymet, within a tolerance. Note that the material used need not strictlybe a polymer; all that is necessary is the negative thermo-opticcoefficient. This material is placed intracavity in order to affect thesummation in equation (5). Since it is the effective index of refractionthat appears in equation (5), it is sufficient that some optical energypropagating in the optical mode traverse the negative coefficientmaterial in one segment of the round trip optical path. The polymermaterial may be used in the cladding or the core, or in both portions ofthe optical waveguide. Since the thermo-optic coefficient of polymerstend to be large, only a fraction of the optical mode volume swept outby the optical mode in a transit of the optical cavity need be occupiedby polymer.

The cavity is preferably also made athermal without changing its FSR. Toaccomplish this objective, the preferred approach is to adjust theoverlap factor and the thermal coefficient of the polymer while keepingthe optical lengths at the values required for the desired FSR. With theabove lengths, a round trip through the diode laser contributes about0.2 micron/° C. to the first term of equation (5). Thermal compensationis achieved by the polymer waveguide if its net thermal coefficient (thechange in the effective index of refraction with increase intemperature) is approximately −9×10⁻⁵ ° C.⁻¹. (The second term ofequation (5) is small.) While the core material in the preferredembodiment has a positive thermal coefficient and is traversed by themost intense part of the beam, the polymer cladding material has such alarge negative coefficient that it can be effective in compensating theentire cavity.

The fraction of the optical mode power in the waveguide that propagatesinside the polymer compensating material is given by the overlap factorΓ_(c)

Γ_(c)=(mode power propagating in the polymer)/(total mode power)  (6)

which may lie in the range of a few tenths of a percent up to 40% ormore, depending on the design of the waveguide core, and the placementof the optical polymer. For the preferred 2 micron square high contrast2% silica waveguide on silica cladding, the exponential tails of themode penetrate far out of the core into the polymer cladding asdescribed below in relation to FIG. 11A. Assuming the index ofrefraction of the polymer has been adjusted in the preferred embodiment(by e.g. halogenation and/or mixing) to equal that of pure silica, theoverlap factor is about Γ=40% since the polymer material forms thecladding on three out of four sides of the rectangular waveguide core. Apolymer material with dn/dT of about −23×10⁻⁵ ° C.⁻¹ will achievethermal compensation of this resonator. Materials with larger negativedn/dT may be used with a design that has proportionately smaller overlapfactor or smaller physical length through the negative dn/dT material.For example, if a material with dn/dT of 34×10⁻⁵ ° C.⁻¹ is used, thedesired overlap factor is reduced to 26% in the above structure.

The tolerance within which equation (5) is satisfied depends on theapplication. In the case of the communications application, a modefrequency shift of a fraction of the communications channel spacing, say5 GHz, may be tolerated over a temperature range of operatingtemperatures, which might be only a fraction of a degree for temperatureregulated devices, or as much as 5° C., 50° C., or even higher forunregulated packages. A 50° C. range would imply a tolerance of about+/−0.001 microns/° C. in equation (5). To achieve this tolerance in areal device, the waveguide lengths and the dn/dT of the polymer arepreferably controlled to an accuracy of a fraction of a percent.Depending on the parameter values, the tolerance on the cavity lengthmay rise to about 10 microns (for a wide 400 GHz channel spacing, forinstance), or it may fall below one micron (for a narrow channelspacing).

When the grating is integrated with the thermally compensated waveguidedesign described above, an advantageous wide tuning range results. Thetuning range of the polymer clad grating 130 or 132 is large bothbecause of the large thermo-optic coefficient and large mode overlapfactor of the polymer. When the temperature of the grating polymer isscanned over a 100° C. range, the grating wavelength tunes overapproximately 9 nm for the above case of polymer material with −23×10⁻⁵° C.⁻¹, and 40% overlap factor.

In operation, the device will settle to a given temperature profilealong the optical path of the resonator. The laser amplifier generatesheat, and will rise to a temperature above that of the polymerwaveguide. Heaters or coolers attached to the device, such as asubstrate heater or the TE cooler described in reference to FIG. 2, mayalso change the temperatures of the gain section and the intracavitywaveguide. Once the device in operation has reached equilibrium, thethermal profile will vary spatially along the waveguide but it will beconstant in time. Changes in ambient temperature will change the entireprofile approximately by a constant amount. Particularly if the thermalconductivity of the substrate is large, such as is the case for thepreferred silicon substrate, changes in ambient temperature will producespatially uniform changes in the thermal profile. Such changes intemperature do not substantially change the mode positions or FSR in anathermal cavity as described by equation (5).

Laser Embodiment

FIG. 1 shows a preferred embodiment of the hybrid tunable laser chip100. A semiconductor laser chip 110 is flip-chip bonded to the substratechip 120 producing a hybrid of two integrated waveguide chips. The laserchip is preferably fabricated from InP so that it emits in the 1550 nmregion or the 1310 nm region. The waveguides 112 and 114 provide opticalamplification when excited by sufficient injection current, over anoperating band of optical frequencies including a desired wavelengthsuch as 1550 nm or 1310, 980, 860, 780, 630, or 500 nm, or anotheruseful wavelength region. For a 1550 nm laser, a typical gain bandwidthwould be about 50 nm (such as from 1520 to 1570 nm or from 1560 to 1610nm), and would overlap a portion of the amplifying bandwidth of theEr-doped fiber amplifier either in the conventional band or one of theextended operating bands. The gain bandwidth may be smaller for lowerinjection current, or as large as 120 nm or more for high injectioncurrent and proper quantum well design. The two waveguides 112 and 114of the laser chip are aligned in the x-z plane to butt couple to twopassive (they provide no gain) waveguides 122 and 124 fabricated on thesubstrate chip. A substantial fraction of the energy emitted from thelaser waveguides 112 and 114 is coupled into the planar integratedwaveguides 122 and 124, where the coupling loss is preferably less than10 dB or even less than 4 dB. Vertical alignment (in the y direction) ofthe laser chip 110 is obtained by controlling the thicknesses of theprocess layers in and on the laser chip and the substrate. Light emittedfrom the laser waveguides 112 and 114 is coupled into the waveguides 122and 124 at the aligned butt coupled coupling region. Alternative gainregions include variations on the active region of semiconductor diodelasers, and fiber lasers, dye lasers, color center lasers, solid statelasers generally, or other amplifying media capable of providing opticalgain over a useful frequency band.

Tapered waveguide segments 126 and 128 may be used to improve thecoupling efficiency between the differently shaped waveguides 112 & 114and 122 & 124. See FIGS. 3, 4, 7A, 7B, and 8.

The waveguides 122 and 124 may be integrated on the substrate 120 by oneof a variety of common fabrication techniques. In the preferredapproach, as is known in the art, silica waveguides are fabricated withlow loss and good reproductability using the flame hydrolysis method. Inflame hydrolysis, layers of particles produced in a flame (silica soot)are deposited onto the surface with a chemical composition determined bythe inputs to the flame. Compaction of the particles into a solid filmis typically accomplished during a subsequent high temperatureconsolidation process. Such waveguides are commercially available invarious index contrasts using Ge doped core material, including 0.4%,0.75% and 2%. Ge doped material has the further advantage of beingsensitive to UV irradiation as is known in the art, allowing patternedregions of increased index of refraction (such a grating) to befabricated by exposure to patterned beams of light. Other dopants arealso known to have light-sensitive index of refraction, which may occuras a result of a change in valence state. Most useful layer thicknessesare available, including core thicknesses in the range of 1 to 10microns and beyond, and cladding thicknesses in the range of a fewmicrons to hundreds of microns, if desired. Channel waveguides can becommercially fabricated according to customer design. Channel waveguidesare typically fabricated by reactive ion etching (RIE) after depositionof the core material on the lower cladding material. The RIE stepremoves the higher index core material outside masked regions whereretention is desired to establish light guiding. Subsequent to thechannel waveguide fabrication, a top cladding of silica may or may notbe applied according to the desires of the customer. If applied, the topcladding material is typically identical to the lower cladding material(pure silica) in index, surrounding the core material on all sides withcladding. In the preferred embodiment, we have selected high contrast,2% waveguide core material, with a 2×2 micron channel dimension.

As an alternative the waveguides may be fabricated from spun-on polymerlayers chemically selected with a raised index for the core layer, andpatterned by RIE. Alternative substrates include InP, GaAs, glass,silica, lithium niobate, lithium tantalate, etc. Alternative waveguidematerials include oxides such as Ta₂O₅, Nb₂O₅, TiO₂, HfO₂, and SiO₂,semiconductors such as silicon, GaAs, InP, polymers, and doped or mixedversions of all of the above materials with various dopants includingphosphorus, hydrogen, titanium, boron, nitrogen, and others. Alternativefabrication methods include indiffusion, sputtering, evaporation, wetand dry etching, laser ablation, bleaching, and others. Many differentwaveguide structures are also available including planar, rectangular,elliptical, ridge, buried ridge, inverted ridge, diffused, air clad,hollow, coated, cladding stripped, 3-layer, 4-layer, 5-layer, etc.Combinations of the above materials, methods, and structures may be usedas long as the process flows are compatible (i.e. do not result indecomposition, delamination, or unacceptable chemical change or physicalmodification of the materials of the semi-processed article), theoptical losses are reasonably low (i.e. below 10 dB/cm for very shortchips and below about 1 dB/cm for longer waveguides), and the transverseindex of refraction profile of the finished structure has a locallyhigher index of refraction compared to adjacent materials in at leastone dimension, creating at least a planar waveguide that guides light inone dimension or a channel waveguide that guides light in twodimensions.

FIG. 11A shows the preferred cross sectional embodiment of thewaveguides 122 and 124 of FIG. 1. The waveguides have a polymerstructure, fabricated with a silica under cladding, a patterned Ge-dopedsilica core, and an over cladding of a polymer material whose index ofrefraction has been selected to approximately equal (within a toleranceof less than about 1.5%) the index of the silica under cladding. Thepreferred polymer material is a deuterated and halogenated polysiloxanesuch as is described in M. Amano, U.S. Pat. No. 5,672,672, Sep. 30,1997, as Compositions (G), (H), (13), (14), (18), (20), (23), or (24).Other polymer materials can also be used, including polysiloxanes,acrylates, polyimides, polycarbonates, etc., with optional deuterationor halogenation to reduce optical losses in the infrared, adjust theindex of refraction, and adjust adhesion to other layers. Depending onthe layer thicknesses, waveguide stripe width, and refractive indices, asubstantial fraction of the mode energy propagates in the polymercladding. This fraction may vary from a very small fraction of a percentup to many tens of percent. The cross section of the waveguide 1100shows the optical mode 1180 represented by intensity contour levelspropagating along a channel waveguide emerging from the page. Thechannel waveguide is fabricated on a silicon substrate 1170 with asilica lower cladding 1144, and an initially-uniform Ge-doped layer hasbeen etched into a square ridge 1140 that forms the core of thewaveguide. Spun on top of the ridge layer is the polymer upper cladding1142, which shows a small bump above the ridge due to incompleteplanarization in the spin and cure process.

The structure 1120 is an electrode. Depending on the nature of thematerials used, its usage and properties are slightly different. For thepreferred thermo-optic polymer device, layer 1142 is a thermo opticpolymer, and the electrode 1120 is a resistive stripe for heating thestructure of the waveguide in a controllable way. If the material 1142is electro-optic such as a poled nonlinear polymer or for instance, thestructure 1120 is an electrode for applying an electric field across thematerial 1142 towards another electrode which may be remote such as onthe rear surface of the substrate or a package wall, or on the topsurface shown but laterally displaced from the electrode 1120.

FIG. 11B shows an alternative waveguide embodiment with split topelectrodes 1122 and 1124, a lower electrode 1126 provided below thelower cladding, and additional upper cladding material 1141. With lowerelectrodes as shown, the actuating voltage of an electro-optic device islowered since the separation between electrodes so disposed is small. Ifthe material 1142 is a polymer dispersed liquid crystal (PDLC) forexample, a split electrode structure may be used to enable applicationof vertically and laterally oriented fields. This enables rotation ofthe applied electric field direction as well as changes in its strength,producing changes in the TE and TM indices of refraction of the PDLCfilm 1142. In further variations, the material 1140, 1144, or 1141 maybe the tunable material with thermo optic, electro-optic, etc.properties.

The waveguides 112 and 114 are preferably curved near the front facet118 of the laser chip so that the butt coupled interfaces lie at anangle to the direction of propagation of light in the waveguides,reducing the feedback from the coupling region. The front facet 118 mayalternatively be antireflection coated, index matched, etc. Thesemeasures diminish the feedback from the front facet relative to thefrequency selective feedback from the gratings 130 and 132, increasingthe stability of the system.

FIG. 5 shows a detailed top view of the coupling region between thediode laser chip 110 and the waveguide 122. The laser waveguidestructure 112 meets the HR-coated rear facet 116 of the diode laser atnearly normal incidence for good coupling of light reflected from thefacet 116 back into the waveguide. However, the waveguide is preferablycurved in the region 510, with a radius of curvature R, so that it meetsthe front facet 118 (which is preferably parallel to the rear facet 116)at an angle θ_(i). The performance of curved waveguide diodes isdescribed in some detail in C-F. Lin et al., IEEE Phot. Tech. Lett. 8,206, (1996). The angle θ_(i) is chosen to be large enough (preferablyabout 8°) so that the reflection from the interface at the facet 118does not re-enter the waveguide, preventing feedback from thisinterface. The minimum desired angle depends on the contrast anddimensions of the waveguide, but as a rough rule of thumb it can bechosen larger than about 50 for a tight waveguide. Since the effectiveoptical index of refraction inside the laser chip is approximately n=3.3and the effective index of the tapered waveguide core 126 is about 1.49,the angle θ_(r) of the output waveguide will be approximately θ_(r)=18°.

As mentioned in reference to FIG. 3, an index matching material ispreferably applied in the gap between the laser chip 110 and thewaveguide structure 126 & 122. The gap is more clearly shown inreference to FIG. 3 between the waveguide 330 and the waveguide 320. Theinterface region includes two interfaces, one between the waveguide 320and the material in the gap, and a second between the material in thegap and the waveguide 330. The gap may be filled with air, vacuum, or amaterial with index of refraction close to but preferably somewhat abovethe geometric mean of the effective indices of the waveguides 320 and330. (If the waveguide structure 122 and 126 is fabricated after theattachment of the diode laser to the substrate there may be no gap atthis interface.)

As an alternative to the butt coupling arrangement between the twowaveguides shown in FIG. 1, many other coupling approaches may be used,including lens coupling, grating coupling, and parallel coupling(including vertical coupling), and grating assisted coupling, as isknown in the art. In the case of vertical coupling, the waveguide 122 isdisposed parallel to and vertically separated from the diode laserwaveguide 112, as would be obtained if the waveguide 122 is fabricateddirectly on top of the diode laser waveguide. The coupling between thetwo waveguides may be that of a broadband directional coupler, or it maybe that of a narrower band grating assisted coupler.

Monitor photodiodes 140 and 142 may be placed to receive a portion ofthe light generated from the laser chip, in this case in proximity tothe rear facet 116 of the laser which has preferably been highreflection (HR) coated, but which still transmits a portion of the lightincident on the facet. As shown, the monitor photodiodes 140 and 142 arepreferably waveguide detectors butt coupled to the laser waveguides 112and 114. This butt coupling is non critical since the laser power ishigh and high detector efficiency is not critical. A large separationbetween the laser and monitor chips on the order of 50 microns or moreis acceptable, making possible reduced positioning tolerance for thischip. If desired, one of many known configurations for the dispositionof surface photodiodes may alternatively be used.

Grating regions 130 and 132 are tunable frequency selective feedbackstructures that reflect a portion of the light traveling in waveguides122 and 124, providing feedback into the laser chips, and determiningthe wavelength regions in which the lasers oscillate (see E. I. Gordon,U.S. Pat. No. 4,786,132, Nov. 22, 1988, “Hybrid distributed Braggreflector laser”). The gratings 130 & 132 and the rear facet 116 of thelaser chip form the cavity mirrors for the laser oscillator of a hybridexternal cavity, grating stabilized laser. The waveguides 112 and 114,the butt coupling regions, and a segment of the waveguides 122 and 124,respectively, including the tapers, form the intracavity optical pathfor propagation of optical energy within the resonator. These gratingregions 130 and 132 are shown in a separate segment of the waveguides122 and 124 from the tapered coupling regions 126 and 128. Theseseparate segments of the waveguide may be identical to or may differfrom the other segments of the waveguide either in structures or inmaterials. The grating structure may be fabricated in the core layer1140, one of the cladding layers 1142, 1142, or 1144, or in multiplelayers.

When the drive current through the laser waveguide 112 exceeds athreshold value, the gain provided exceeds the round trip optical lossof the oscillator, and laser operation is obtained. The FSR of thelinear cavity of FIG. 1 is determined by the optical length between thefirst grating element and the rear facet 116 of the laser amplifierchip. The partial waves of the reflections from the other gratingelements add in phase to determine the spectral characteristics of thegrating reflection. Together, these partial waves also establish theamplitude and phase of the grating reflection coefficient at thelocation of the first grating element. Changes in index within thegrating affect its spectrum but not the FSR of the cavity, whether thosechanges in index are uniform or have a complicated profile due toheating, the application of an electric field to electro-optic material,the application of stress, etc.

For single frequency operation as is required for high bandwidthcommunications, the width of the grating interaction band is preferablymuch smaller than the gain bandwidth of the amplifier but comparable tothe FSR of the resonator containing the amplifying waveguide. If thefull width at half maximum of the main grating reflection band equalsthe longitudinal mode spacing, only one mode at a time will lase.Adjacent modes will have lower gain, and will be clamped below thresholduntil the grating band is tuned far enough to equalize the modal gain oftwo adjacent modes. When gain equality is obtained for two modes duringtuning, the operating frequency of the laser will jump suddenly from theoscillating mode to the adjacent mode the grating is tuning towards. Itmay be sufficient for the grating band full width at half maximum to besubstantially larger than the FSR, but the laser stability will begin tobecome compromised as the band width becomes significantly larger. Thereare some advantages to having the band width smaller than thelongitudinal mode spacing, but the laser might become power modulated oreven extinguished as tuning progresses.

By modulating the drive current, the laser intensity may be modulated,thereby modulating the output power coupled out of the waveguides 122and 124 into the output fibers. The waveguides 122 and 124 may bemodulated with separate data, providing multiple independent outputchannels, or simultaneously with the same data stream, providing a dualoutput device that can be independently routed to the desireddestination for e.g. data communication protection purposes. Lasermodulation may be accomplished by modulating the drive current throughthe stripe waveguides 122 and 124, or externally as is known in the art.Direct modulation is accomplished with low chirp in this configurationbecause changes in the drive current do not modulate the index ofrefraction of the gratings 130 or 132, and because the effective indexof most of the optical path of the cavity is not modulated. The maximumrate of modulation is typically limited by the round trip time in thecavity to a fraction of the FSR, so if the FSR is set at 50 GHz, themodulation rate may be limited to 10 to 20 GHz.

Since the amplifier chip generates heat, changes in the average drivecurrent will also change the longitudinal mode positions. It istherefore preferable to establish an average drive current that ismaintained during operation. A constant drive current may be maintainedduring modulation using a transition-keyed modulation scheme, forexample, if necessary. As the laser ages, its average drive current mustrise to maintain constant output power and good modulationcharacteristics. It may be desirable to apply a compensating level ofcurrent to intracavity electrodes such as 150 and 152, for example, tomaintain the channel frequencies despite the ageing-related rise inlaser temperature.

The grating regions 130 and 132 are preferably fabricated by patternedexposure through a phase mask as is known in the art. The grating may befabricated in the Ge-doped silica core after sensitization with hydrogenor deuterium, or it may be fabricated in the polymer cladding prior tofull cure so that the cladding material is still subject to chemicalchange such as by crosslinking. Alternative grating fabrication methodsinclude exposure with interfering beams, patterning and dry- orwet-etching, or direct patterned etching, all of which are known in theart and may be applied to either core material or cladding material. Fora purely periodic grating, the Bragg wavelength λ_(B) for peakreflection in the retroreflecting configuration shown is given by

λ_(B)=2n_(eff)Λ/m,  (7)

where Λ is the grating period, n_(eff) is the local effective index ofrefraction of the mode, and m is the order of reflection. The result inequation (7) follows from the requirement that to accomplishphasematching, the wavenumber of the grating 2π/Λ must equal the sum ofthe forward propagating wavenumber in the waveguide and the reversepropagating wavenumber in the waveguide. With an effective index ofabout 1.446, Bragg wavelengths of 1552 nm and 1310 nm are obtained withgrating periods of 537 nm and 453 nm, respectively. The exact wavelengthof operation depends on all of the optical parameters of the waveguide,including the grating periods, and the refractive indices andthicknesses of the films traversed by the optical energy of the opticalmode.

In practical devices, gratings are rarely strictly periodic, and thegrating period, the grating index modulation, and the waveguideeffective index of refraction may be varied along the length of thewaveguide to achieve various effects such as apodization as is known toreduce sidemode reflection, to create multiple grating peaks as knownfor example in superstructure gratings and sampled gratings etc., or ingeneral to engineer the shape of the reflection spectrum. In a laserresonator (or oscillator or cavity), it is preferable to chirp thegrating period towards shorter period (in the direction of lightpropagation away from the amplifying waveguide segment) both forenhanced oscillator stability as shown by P. A. Morton et al., Appl.Phys. Lett. 64 2634 (1994), and for reduced sidelobe amplitude on thegrating reflection bands as shown by A. Gnazzo et al., IntegratedPhotonics Research Conference, Optical Society of America, p. 410(1996). The waveguide parameters such as lateral guide width may also bespatially varied, changing the effective index and the grating frequencyas is known in the art.

The optical frequency band over which reflection occurs may take on verydifferent forms according to the shape of the grating spectrum which mayhave only a single narrow peak, a broad peak, or a more complexmultipeaked structure. The shape of the spectrum depends on the detaileddesign of the optical phase advance along the grating structure. Asingle peaked grating may be used to select a single channel, and abroad band or multipeaked grating may be used to select multiplechannels or provide vernier tuning, etc.

The grating may be tuned (shifting the wavelength range for interactionwith light) by changing either the grating period or the effective indexof refraction of the light propagating through the grating. The gratingperiod may be changed by expanding the material by one or more ofseveral means including mechanical stretching or compression, heating orcooling, acoustic excitation, etc. The effective index in the gratingmay be changed by one or more of several means including the preferredthermo-optic effect, the electro-optic effect, the piezoelectric effect,etc. Materials are available that change their index of refraction inresponse to thermal, electric field, compression, shear, and otherapplied changes, including nonlinear optical materials, crystals, liquidcrystals, and other types of material known in the art. Any portion ofthe material traversed by the optical energy of the light mode along thegrating may be changed to affect a change in the grating spectrum. Thethermo-optic effect is preferred for shifting the reflection band in anear-term product due to the availability of reliable polymer materialswith large dn/dT.

The thermo-optic effect is the property of some materials of changingtheir index of refraction with temperature. Heating a segment of thepolymer waveguide of FIG. 11A changes the effective index of refractionpredominantly through the thermo-optic effect. The effect of thermalexpansion is relatively small. A few materials have large thermo-opticcoefficient (dn/dT) such as the active waveguide of the InP laser(dn/dT=25×10⁻⁵ ° C.⁻¹), and a few materials have a small rate of changeof index with temperature such as silica (dn/dT=1×10⁻⁵ ° C.⁻¹). Polymermaterials are unusual in that their thermo-optic coefficient is negativeand large (dn/dT in the range of −10 to −35×10⁻⁵ ° C.⁻¹), see forexample R. S. Moshrefzadeh et al., J. Lightwave Tech. 10 420 (1992). Inthis invention, we use polymer layers along the optical path to tunegrating interaction frequencies, to tune resonant frequencies, and torender devices athermal by compensating the positive thermal change inindex of refraction of other materials traversed by the optical energy.Useful devices are produced including in particular the combination ofboth thermally insensitive structures such as resonators, and stronglythermally tunable structures such as polymer gratings.

Changes in the temperature of the grating do not affect the FSRsubstantially if there is no substantial “leakage” of the gratingthermal spatial profile into the resonant cavity. The cavity as a wholeis in the preferred embodiment made athermal (compensated to beinsensitive to uniform temperature changes). However, individualsegments within the cavity may still have a substantial thermalcoefficient. In the preferred embodiment, the structure of the waveguide122 is the same in the grating 130 and in the cavity between the taper126 and the grating 130. To the extent that a portion of the thermalspatial profile from the heater 160 overlaps the laser cavity, tuningthe grating with heater 160 will still produce a residual change in themode frequencies. Ideally, the temperature tuning of the grating isaccomplished by an abrupt spatial thermal profile that changes thetemperature of the grating but that does not change the temperature ofthe intracavity waveguide structure. By designing the heater electrodes160 and 162 for low heating of the intracavity waveguide region outsidethe length of the grating, and by providing a high thermal conductivitysubstrate 120 such as silicon, and by keeping the thickness of theprocess layers thin between the grating waveguide and the substrate(subject to other constraints), we can minimize the effect of thethermal tuning of the grating on the longitudinal modes, so that thefull tuning range of the grating can be realized while limiting theundesired mode tuning to a tolerance such as an acceptably smallfraction of one FSR. As an alternative, a segment of the intracavitywaveguide adjacent to the grating could be designed athermal in theregion of “leakage” of the grating thermal profile.

Under such conditions, tuning the Bragg wavelengths of the gratings 130and 132 by means of the currents flowing through the heater stripes 160and 162 produces a series of discrete frequency jumps in the laseroutput (mode hops) from one longitudinal mode to the next, withoutchanging the longitudinal mode frequencies. The optical frequency ofoperation tunes in a discontinuous, digital manner, without traversingthe frequency range between the longitudinal modes of the cavity. Iflongitudinal modes coincide with communications frequency channels, thedevice changes communications channels digitally even though the currentin the heater stripes may be changed continuously in an analog fashion.With digital tuning, the channel accuracy depends not on the accuracy ofthe tuning actuator (e.g. heater current), but on the accuracy of thespecification of the channel frequencies.

A pair of serpentine heater traces 150 & 152 may be disposed about thewaveguides 122 & 124, at a location between the grating regions 130 &132 and the output facet 118. The heater traces 150 and 152 terminate inelectrodes 154 and 156, and 158 and 159, respectively. Injecting acurrent through the electrode pair 154 and 156 excites the heater trace150, raising the temperature of the waveguide 122 along a portion of itslength as determined by the pattern of the heater trace 150 and thediffusion of the heat away from the trace and (ultimately) into thesubstrate. Likewise, injecting a current through the electrode pair 158and 159 excites the heater trace 152, raising the temperature of aportion of the waveguide 124. The heater traces may be fabricated fromstripes of resistive material such as platinum, nickel, Nichrome,conductive polymer, etc., and may be in the form of a single layer or ofmultiple layers as may be necessary to produce the desired properties ofconduction or wirebonding or adhesion to the lower layer, or to modifythe electrode response to subsequent process steps such as laserablation, etching, etc. The stripe may be patterned as known in the artby lithographic means such as photo resist patterning followed by liquidor dry etch (e.g. chemical or RIE etch) of the resistive material andstripping of the resist. These electrodes and heater traces may be usedto adjust the optical length of the round trip optical path of thehybrid external cavity grating stabilized laser, where the round tripoptical path is the path followed by the optical mode through theresonator between successive passages through the same point in phasespace in the resonator (such as a reflection from the grating or acoupling into an amplifying waveguide segment), and traversing theamplifier waveguide segment, and where n_(i eff) is the effective indexof refraction of the mode in the i^(th) segment traversed by the lightin the cavity, L_(i) is the physical length of the i^(th) segment, andthe sum is over all the path segments traversed. The heaters lower theeffective index of the waveguides through the thermo-optic effect in thepolymer cladding material in the region determined by the heat flowadjacent to the heaters. This reduces the optical length of theresonator, increases the FSR and tunes the longitudinal modes to higherfrequencies, all other factors being constant.

In an embodiment of the invention, the round trip optical length may beadjusted by means of the heaters 150 and 152 to adjust the opticallength and the free spectral range so that some of the resonatorlongitudinal mode frequencies coincide with a desired set ofcommunications frequency channels. Or, the heaters 150 and 152 may beused to tune the operating frequency of the device in a continuousanalog fashion.

Serpentine heater traces 160 & 162 may be disposed about the waveguides122 & 124, at a location within the grating regions 130 & 132 andsubstantially traversing the entire grating regions. The heater traces160 and 162 terminate in electrodes 164, 166, 168, and 169. Injecting acurrent through the electrode pair 164 and 166 excites the heater trace160, raising the temperature of the waveguide 122 along grating region130 as determined by the pattern of the heater trace 160, and thediffusion of the heat away from the trace and (ultimately) into thesubstrate. Again, the heaters lower the effective index of thewaveguides through the thermo-optic effect in the polymer claddingmaterial in the region determined by the heat flow adjacent to theheaters. The change in the effective index in the grating region tunesthe frequency response of the grating as in equation (7); heating agrating segment increases its frequency of interaction. The gratings maybe tuned together or separately simply by controlling the respectiveheater currents or powers.

The stripe pattern of the heater traces 160 and 162 is preferablyuniform along the grating to form a thermal change as a function ofheater current that is uniform along the length of the grating, therebylargely maintaining the spectral shape of the grating interaction. Asshown, the stripe pattern traverses both sides of the waveguide in thegrating region so that the thermal change is also more uniform acrossthe lateral dimension of the waveguide. Use of a single heater stripealong the waveguide is a reasonable alternative that offers theadvantage that all the gratings may be grounded together at one end.

Although the heater stripes 150 and 152 are also shown as serpentine,uniformity is not a requirement for tuning the round trip optical lengthof the resonator. The electrodes or pads 154, 156, 158, 159, 164, 166,168, and 169 are preferably made of gold or other material that resistsoxidation in order to enhance the bonding of connection leads to theheater power supplies (not shown). The locations of these electrodes arenot critical, and may be moved to other locations on the chip, providedthat the connections between the electrode locations and the heatertraces have low resistance to reduce unwanted power consumption. Manyother heater and electrode designs are available and useful foraccomplishing the purposes described above.

In a variation of the invention, the thermal profile along the waveguideinduced by the heater stripes may be made nonuniform along the length ofthe gratings by various means including varying the width of thestripes, varying the distance of the stripes from the waveguide axis,etc., so that the spectral shape of the grating interaction may bechanged by a distributed thermally induced phase shift as a function ofthe heater current.

For thermo-optically tunable gratings, while the cavity may be madeathermal, the grating itself cannot be athermal. For this reason, it maybe desirable to stabilize the absolute temperature of the substrate,limiting the frequency sensitivity of the grating to changes in ambienttemperature. If the substrate is thermally stabilized, the heater powerprovided to the grating may also be used to determine the absoluteoperating frequency. Some drift in the grating frequency is acceptableprovided it does not cause a mode hop, so the substrate stabilizationrequirement is not very stringent. (In electro-optic, piezoelectric,etc. devices, the gratings are preferably designed to be intrinsicallyathermal, eliminating the need for substrate thermal stabilization.) Tostabilize the substrate temperature, a simple temperature sensor may beattached at or near the substrate with an electronic control feedbackloop provided as is known in the art to actuate a heater and/or cooler(such as the TE cooler 212) and regulate the temperature within adesired range.

A curved waveguide region 178 may be provided in the waveguides 122 and124 on the chip 100 to bend the waveguides back through the angle θ_(r)to provide output coupling to a set of optical fibers that is parallelto the diode laser chip, allowing easy scaling of the design to multiplelasers on the same chip. By expanding the chip laterally (in thex-direction), a wider laser chip with 3 or 4 or more waveguides can beprovided and coupled to additional waveguides laid out adjacent to theexisting waveguides, with taper, grating, and heater sections, as wellas output fiber V-grooves. The dual-bend configuration allows this to bedone with identical 25 length segments for each separate waveguide. Theradius of the curved waveguides in the region 178 may be chosen tooptimize the bend loss; a good choice for our 2% contrast silicawaveguides is a radius of curvature larger than or equal to about 2 mm.Notice that the bends have been placed outside of the resonant cavity toreduce the length of the resonant cavity (increasing its modulationbandwidth) and to reduce its loss. An alternative design (not shown)incorporating this bend before or after the taper but before the grating(and therefore inside the cavity) has the advantage of greatercompactness since the grating regions will also be parallel with thelaser chip.

V-grooves 170 and 172 may be provided to aid in coupling a pair offibers (not shown) to the output ends of the waveguides 122 and 124. TheV-grooves extend across the bonding slot 176 and terminate in thealignment slot 174 whose vertical sidewall allows the butt coupling ofthe output fibers and the waveguides 122 and 124. The depth and positionof the V-grooves are adjusted to align the core of the output fibersapproximately coaxial with the waveguides 122 and 124 at the alignmentslot 174.

FIG. 6 shows a detailed top view of the coupling region between thewaveguide 122 and the V-groove 170. Alignment slot 174 is preferablyfabricated with a nearly vertical sidewall 610 in which the waveguidecore 122 terminates. A fiber is placed in the V-groove 170, gentlypressed against the two angled sides of the V-groove, and gently pressedforward against the sidewall 610. Adhesive is placed in the region 179(see FIG. 1) and cured, to affix the fibers in position. The bondingslot 176 prevents adhesive from wicking along the fibers or V-groovetowards the optical interface at the sidewall 610. The position andangle of the V-groove in the x-z plane, and the depth of the V-grooveare preferably set so that the attached fibers are aligned coaxial withthe waveguide core 122. The fabrication of V-grooves in silicon, silica,and other substrates is known in the art, as is the slot design and thefiber attachment process, providing multiple alternative realizations.

The electrical connections to the common connection to the back surfaceof the laser, and for the laser diode stripes, are preferably made viawirebond connections to intermediate electrodes 181, 182, and 183, . . ., respectively. Connections to the monitor photodiodes 140 and 142 mayalso be made via wirebonds to connection pads (or electrodes) such as184, 185, . . . , possibly also using 181 as a common connection. Ifcommon connections are electrically undesirable, as may happen in someelectronic circuits sensitive to noise, separate electrodes may be usedfor each common function. For example, the monitor diodes may have oneor even two separate common electrodes (not shown).

FIG. 2 shows a cutaway view of a package arrangement 200 for the hybridchip assembly 100. The chip assembly 100 is preferably bonded at itslower surface 102 to two thermo electric coolers 212 and 214 bonded inseries. Two coolers are preferably used to enable a large temperaturedifference between the chip 100 and the ambient temperature, makingpossible a wide ambient temperature range over which operation can beobtained while maintaining the chip 100 within its desired narrowtemperature range. For broader or narrower operating ranges, more orfewer coolers may be used. If the range of ambient temperature excursionis as small as 5° C. or so, no cooler may be necessary. The TE coolersare in turn bonded to a heat sink 220 shown as part of the package madefrom one of several appropriate thermally conductive materials includingcopper, aluminum, Kovar, ceramic, etc. This heat sink may have fins orbe attached to fins (not shown) for improved heat conduction into theambient air. Electrical connections 230-233 are shown between the chipand the leads of the package 234 which emerge through isolating regions236. The connections 230-233 may be directly to electrodes on the chipsuch as electrodes 164-169, or directly or indirectly (via electrodes181-185 . . . ) to electrode regions on hybrid integrated elements suchas the monitor diodes 140 or 142 or the laser diodes 112 and 114. Thefibers connected to the chip assembly 100 emerge through the packageeither via connectors or seals (not shown). The sealing plate 240 may besoldered or welded to the rest of the package if a hermetic seal isdesired to exclude humidity for example, or it may sealed with adhesive,or even replaced with a potting material if hermeticity is not required.

The frequency selective feedback structure may alternately be a gratingassisted coupler in codirectional coupling or reflective coupling, abulk-optics grating, a resonator or etalon either in bulk form or in awaveguide (as for instance fabricated by etching two parallel facets ortrenches across a waveguide to form a waveguide Fresnel reflector), orother devices capable of selecting a spectrum within the opticalfrequency range that is then fed back into the amplifier medium by meansof optical structures including bulk optics, waveguides, or otherintegrated optical components. Some of these structures are compatiblewith a ring laser embodiment including simple rings, multiple rings withinterconnections, and more complex topologies in three dimensionalwaveguide structures.

Ring Laser Embodiment

FIG. 9 shows a ring resonator structure embodiment 900 which is analternative to the linear resonator structure embodiment 100 of FIG. 1.The optical radiation emitted from the two facets 916 and 918 at eitherend of the amplifier waveguide 912 in the amplifier chip 910 ispreferably butt coupled to waveguides 922 and 924, respectively. Theamplifier chip 910 is preferably flip chip bonded to the substrate 970on which the waveguides 922 and 924 are integrated. The waveguides 922and 924 curve into a loop, passing each other closely in a parallelcoupler region where their transverse mode profiles overlap but theireffective indices n_(eff1) and n_(eff2) are preferably dissimilar enoughto produce negligible coupling. Antireflection means are provided at thefacets 916 and 918 from among the alternatives described above, asbefore including preferably angled waveguides at the facets, to reduceoptical feedback and suppress lasing between the facets. An opticalgrating 930 overlaps optical energy flowing through the two waveguides,and provides the phase matching that allows coupling between the opticalmodes of the waveguides, allowing laser feedback to occur around thering resonator formed by the two waveguides, coupler, and amplifierwaveguide.

The grating period Λ of the grating 930 needed to accomplish thiscoupling follows from the phasematching requirement that the wavenumber2π/Λ of the grating must equal the difference in wavenumbers of the twowaveguides, or

Λ=λ/¦n_(eff1)−n_(eff2)¦ (codirectional coupler)  (8)

When Λ approximately satisfies equation (8), the light traveling in onewaveguide is coupled across the parallel coupler to the other waveguide,and the direction of propagation of the light is maintained in the samesense of propagation around the ring. For example, light emitted fromthe facet 918 is coupled into the waveguide 924, then across into thewaveguide 922 by the grating 930, where it travels back towards thelaser chip 910, is coupled back into the amplifier waveguide 912 at thefacet 916. This light is amplified and emitted again at the facet 918,having made a round trip of the ring resonator. Light emitted from thefacet 916 is coupled into waveguide 922 and travels in the oppositedirection around the loop, so the ring resonator lases bidirectionallyunless a unidirectional element (not shown) is introduced. Some of thelight traveling in the waveguides 922 and 924 may be transmitted throughthe grating region and remain in its respective waveguide. Thistransmitted light is conducted by its respective waveguide to an outputsurface such as 925, where it emerges for use. The structures and usagesdescribed in respect of the individual embodiments herein also apply tothe other embodiments. Therefore, for example, V-grooves may be used toalign output fibers to the output ends of the waveguides at the outputsurface 925, adjustments may be applied to the optical length of theresonator to bring the FSR to equal a rational fraction of acommunications channel spacing, a polymer material and heater stripesmay be used within the grating region to tune the grating to coupledifferent cavity modes between the waveguides 916 and 918, etc.

The ring resonator of FIG. 9 lases when the gain in the amplifier issufficient to overcome the losses in the waveguide ring resonator. Theloss will be lowest for the longitudinal mode of the cavity withfrequency closest to the peak frequency of the grating interaction. Evenif the suppression of the gain for the adjacent modes is small comparedto the favored mode, the laser will oscillate on the favored mode. Toobtain a good sidemode suppression ratio such as 30 to 50 dB, however, asignificant difference in gain between adjacent modes is desired, so thegrating is preferably designed with a substantial additional loss forall other modes. If the injection current of the laser chip is modulatedto transmit data, two almost identical outputs are provided that aremodulated with the same data. This characteristic may be advantageous insystems requiring a backup transmission line in case of network failureon one of the lines. The output from the two waveguides 922 and 924 maybe identical but for the additional bending loss experienced by thewaveguide 924.

The above is a specific implementation of the general situation in whichlight from an amplifier waveguide follows a first path to couple with agrating structure and then returns to the amplifier following a secondpath, the optical path forming a closed loop figure in two or threedimensions. The entire loop in FIG. 9 is comprised of waveguidesegments, and the grating structure is a grating assisted parallelcoupler within the loop. The ring resonator of FIG. 9 is preferably madeathermal and its modes adjusted to coincide with specified frequencychannels. For a tunable grating device, it is preferable for the tuningmechanism to leave unchanged the cavity FSR, while tuning the gratinginteraction frequency. The accomplishment of these objectives is morecomplicated in this case since the grating lies intracavity. Thepreferred approach is to make the thermal coefficients equal andopposite for the two adjacent waveguides in the grating region 930. Anon resonance mode that couples across the grating assisted parallelcoupler will traverse the same path length on each side of the coupler.The thermal dependence of the two waveguide segments then compensateeach other when the grating temperature is varied symmetrically aboutthe center of the grating. Since it is the difference in propagationconstants that tunes the grating in a co-directional coupler, equallengths of opposite thermo-optic coefficients add to produce a netthermal tuning sensitivity.

Equal and opposite thermal coefficients in a pair of waveguides may beaccomplished with one of the structures described in reference to FIGS.14C and 15. By adjusting the thickness of the layer 1542, or the depthof the etch shown in step 14B, the amount of overlap of the left mode(e.g. 1482) in the polymer material 1442 or 1546 may be adjusted. Asecond adjustable design parameter is provided in the case of FIG. 14Cby the thickness of the layer 1441, and in both cases by thethermo-optic coefficient of the material 1442 or 1546. Adjusting theselinearly independent parameters is sufficient to bring the thermo-opticcoefficients of the two waveguides to the desired values, whether equaland opposite, or zero and negative, or some other useful combination.

An alternative method of rendering the tuning of the device 900 athermalis to apply a simultaneous heating input to the semiconductor dioderegion 910. The opposite thermal coefficient of the semiconductoramplifier results in cancellation of the effects of the gratingwaveguides on the FSR during tuning provided that the relative amountsof heat input are adjusted to produce equal and opposite optical lengthchanges. A wide tuning range may preferably be obtained in this case bymaking one of the waveguides in the grating region athermal while theother has a maximum negative dn/dT.

FIG. 10 shows an alternative implementation 1000 wherein the grating inthe parallel coupler is a reflective grating and the reflected light iscoupled over to the second waveguide, which then spatially diverges fromthe first waveguide, bringing the reflected light back into theamplifier waveguide segment via a different optical path. Reflectivegratings typically offer narrower bandwidth which can be advantageous inproducing high side mode suppression ratio. In a similar way to FIG. 9,the optical radiation emitted from the two facets 1016 and 1018 ateither end of the amplifier waveguide 1012 in the amplifier chip 1010 iscoupled to waveguides 1022 and 1024, respectively, with antireflectionmeans. The amplifier chip 1010 is flip chip bonded to the substrate1070. The waveguides 1022 and 1024 curve into a loop, passing each otherclosely in a parallel coupler region where their transverse modeprofiles overlap but their effective indices n_(eff1) and n_(eff2) arepreferably dissimilar enough to produce negligible coupling. An opticalgrating 1030 overlaps optical energy flowing through the two waveguides,and provides the phase matching that allows coupling between the opticalmodes of the waveguides. The grating period A of the grating 1030 neededto accomplish this coupling follows from the phasematching requirementthat the wavenumber of the grating 2π/Λ must equal the sum of thewavenumbers of the two waveguides, or

Λ=λ/(n_(eff1)+n_(eff2)) (contradirectional coupler)  (9)

When Λ approximately satisfies equation (9), the light traveling in onewaveguide is reflected and coupled across the parallel coupler to theother waveguide. Again, the direction of propagation of the light ismaintained in the same sense of propagation around the ring. Forexample, light emitted from the facet 1018 is coupled into the waveguide1024, then across into the waveguide 1022 by the grating, where ittravels back towards the laser chip 1010, is coupled back into theamplifier waveguide 1012 at the facet 1016. This light is amplified andemitted again at the facet 1018, having made a round trip of the ringresonator. Light emitted from the facet 1016 is coupled into waveguide1022 and travels in the opposite direction around the loop, so the ringresonator lases when pumped above threshold.

Some of the light traveling in the waveguides 1022 and 1024 istransmitted through the grating region and remains in its respectivewaveguide. This transmitted light is conducted by its respectivewaveguide to an output surface 1025, where it emerges for use. Thelasing characteristics of the device 1000 are similar to those of thedevice 900. As always, the structures and usages described in referenceto the other figures may also be applied in various alternativeembodiments of the devices in FIGS. 9 and 10. The thermal coefficientsof the two adjacent waveguides in the grating region 1030 are preferablyequal to each other. Temperature tuning the grating does not change theoptical length of the resonator if the thermal profile for tuning thegrating ends abruptly at the edge of the grating. Since it is the sum ofpropagation constants that tunes the grating in a contra-directionalcoupler, equal thermo-optic coefficients in equal length waveguide armscombine to give a net tuning range.

Method of Adjusting a Resonant Cavity

Due to fabrication tolerances, the FSR of a batch of devices will varywith a mean and standard deviation that may not allow a high productionyield. A method is needed that allows the FSR of individual devices tobe adjusted to the desired value. A further method is needed that allowsthe absolute frequency of longitudinal modes to be adjusted to coincidewith (fall within the acceptance band of) one of the assigned frequencychannels.

FIGS. 12A, 12B, and 12C, in another aspect of the present invention,show methods of adjusting the resonant cavity, including exposingGe-doped silica, crosslinking a polymer, ablating intracavity material,and depositing additional material in a region traversed by intracavityoptical energy, and including a method of measuring and applying thesemethods to produce the desired communications FSR within one tolerance,and to overlap a desired optical frequency within another tolerance.

FIG. 12A shows the preferred method of adjusting the FSR to the desiredvalue. The waveguide 1240 is illuminated through optical system 1281with electromagnetic radiation 1282. In response to an exposure, thematerial of the waveguide 1240 changes its index of refraction, changingthe optical length between the reflecting facet of laser chip 1210 andthe grating 1230. Verdiell, U.S. Pat. No. 5,870,417, Feb. 9, 1999, hasshown UV illumination of an intracavity silica fiber being used to shiftthe longitudinal modes of a resonator. In this invention, the FSR mustfirst be brought within tolerance before the longitudinal modes can betuned to a desired location. Many materials respond to electromagneticradiation by changing their index of refraction, and the frequency ofthe electromagnetic radiation may be low or high according to themechanism of the interaction (which may include heating with any of thefrequencies, and other effects such as structural change or chemicalchange). It is known that polymer mixtures containing photoinitiatormolecules will cross-link at C═C double bonds, therefore graduallychanging index of refraction when exposed to visible or UV light withsufficient photon energy to activate the photoinitiator molecule. Othermaterials also change index with exposure, including silica, forexample, which in the presence of the Ge dopant will gradually increasein index when exposed to an energy beam of ultraviolet light, and dyedoped polymers wherein the exposure induces a chemical change such as ableaching or molecular configuration change in the dye. Whatever themechanism may be for the radiation-induced intracavity change in indexof refraction, these methods may be applied to change the FSR of theresonator, either increasing or decreasing it according to the nature ofthe material modification that is being induced.

The use of ultraviolet or visible light is advantageous in that theprojected energy beam may pass through several layers of the devicestructure, enabling change in index of material that is buriedunderneath other layers. For example, a suitably prepared Ge:SiO₂ coremay have its index adjusted by UV radiation even though it is buriedunderneath a polymer cladding or protective layer, provided thatsufficient UV radiation reaches the core material. In another example, acrosslinkable polymer layer such as a top cladding layer may be indexadjusted by UV or visible light exposure through an upper protectivelayer that could have a different composition. Or, if there is no upperprotective layer, the lower part of the polymer cladding layer, throughwhich a portion of the optical radiation passes, may be index adjustedby light that passes through the upper portions of the cladding layerwhich are remote from the optical mode.

To achieve the desired FSR in the cavity by correcting it with UVexposure of Ge-doped silica (that decreases the FSR with progressiveexposure), the fabrication parameters prior to correction may be chosento produce a target FSR that is slightly larger than the desired valueso that the main part of the statistical distribution of FSR infabricated but uncorrected devices lies above the desired FSR. Thedirection of the pre-correction deviation of the target FSR ispreferably chosen opposite from the sense of the correction technique,thereby allowing a single technique to be used (in this case increasingthe intracavity index) to produce the desired FSR in this main part ofthe distribution. Ideally, the target FSR differs from the desiredending value by a mean amount comparable to or larger than the standarddeviation of the FSR so that most of the distribution lies on the sameside of the target value. The target FSR is also preferably chosen todiffer from the desired FSR by not more than the amount of change in FSRthat can conveniently be accomplished by applying the exposure. Goodcontrol of FSR correction may be accomplished by making successivemeasurements of the FSR of an individual resonator while applyingsuccessive exposure steps, each exposure being smaller than the totalexposure needed for full correction. The energy in the exposure beam maybe adjusted on each step according to the desired amount of opticallength adjustment for that step. The spatial region being exposed mayalso be changed during the adjusting process, which has the advantage ofallowing a larger net change in optical length, and a greater precision.The free spectral range may be measured by measuring the opticalspectrum of the laser for two or more longitudinal modes, and thismethod may be repeated until the FSR of the cavity equals a rationalfraction of a communications channel spacing within a tolerance such asa few percent of the channel spacing.

To achieve the desired overlap of a subset of longitudinal modes withassigned frequency channels, after the FSR has been set to the desiredrational fraction, the frequency of one of the modes is shifted tocoincide with the desired assigned channel. If the free spectral rangeis equal to the assigned communications channel spacing, and onelongitudinal mode has been aligned to coincide with one of the channels,all longitudinal modes will coincide with assigned channels within afrequency range of linearity.

FIG. 16 shows the method of adjusting the cavity which consists of thesteps shown in the flowchart 1600. The amplifier section is preferablyoperated in step 1610, preferably above the threshold of the resonator,in which case there is a narrowing of the spectrum around thelongitudinal modes of the resonator. There is an optical output that canbe used to determine the longitudinal modes either above or below thethreshold, but the laser is preferably operated above threshold sinceboth the output power and the spectrum depend on the drive current. In avariation of this method, step 1610 is modified to become the step ofoperating a light source to illuminate the optical path of theresonator.

In step 1615 the optical output from the cavity is monitored and in step1620 its spectral features are determined. Specifically, the centerfrequencies of at least two longitudinal modes are identified in 1620 inorder to determine the FSR. This step can be accomplished either aboveor below threshold. In step 1640 the refractive index of the resonatoris modified after calculating the desired rate or duration of theexposure in step 1635. The rate and duration of exposure are preferablycalculated based on the information produced from steps 1615 and 1620.The modifying step 1640 may be accomplished in parallel with step 1615to ensure there will be no overshoot, and steps 1620 and 1635 may alsobe accomplished simultaneously. In step 1635, the process is adjusted(e.g. the rate and duration of the process) so that the measured FSRapproaches and eventually equals the desired FSR within a high degree ofaccuracy. When it is determined that the target FSR is being approached,the modification rate is preferably decreased, and when it is determinedin step 1625 that the target FSR is achieved within a tolerance, theoptical length modification may be stopped in step 1630.

FIG. 12B shows an alternative method of adjusting the FSR by removingmaterial traversed by the mode in the laser cavity. Methods that removematerial include the preferred laser ablation, but also ion beam etchingand other techniques. It is known that many materials that absorb UVlight will ablate when exposed to a sufficiently high short pulseexposure of milliJoules or Joules per square centimeter. The energy beam1283 that is projected onto the intracavity waveguide is preferably anexcimer laser beam that may be directed, focused, or imaged onto thesurface after passing through a mask in the optical system 1281. Eachlaser pulse typically ablates a few tenths of a micron of material fromthe air/material interface, over a spatial extent that may be defined bythe laser beam or by the mask. After ablation, the volume previouslyoccupied with solid material is usually occupied by air or a clearinggas, which has a much lower index of refraction than most solidmaterials. The result is a lowering of the effective index of refractionof the portion of the waveguide where ablation occurs. (Of course, alater deposition of material filling the removed volume can result in alater increase in effective index of refraction.) The laser ablationtechnique is applied to a portion of the material of the cavity that isprepared to lie at a surface accessible to the ablating beam, and thatis traversed by optical energy circulating in the cavity. In FIG. 1, thelaser ablation may be applied to the regions directly over thewaveguides 122 and 124 between the heater electrodes 150 and 152, forinstance, or between the heater electrode region and the taperedwaveguide region, or in the region of the tapered waveguides 126 and128, etc. When material is removed at these surfaces, the effectiveindex is reduced for the mode that traverses the optical cavityunderneath that surface, increasing the FSR. (The ablation technique mayalso be applied to the semiconductor diode laser in that GaAs is knownto ablate, but there may be no large surface on the diode laser that isconveniently available for ablation without disrupting electricalcontacts, passivation, or some other function.)

While the originally prepared surface of the material may be far fromthe optical mode, by proceeding with the ablation process, the desireddeeper surface may be revealed. If a large change in effective index isrequired for a given resonator, the laser ablation process may proceedto a relatively greater depth, closer to the core of the waveguide wherethe mode intensity is highest. For a small change in effective index, ashallow laser ablation process is arranged.

Since a high accuracy may be required even when large changes areneeded, multiple regions of ablation may be used. A deep ablation may beused to accomplish most of the correction, and a separate, shallow,ablation can be performed at a different location on the same resonator.The shallow ablation provides a small change in the effective index fora large exposure pulse, allowing high precision to be obtained inadjustments to the FSR. A further method of increasing the precision ofadjustment is to reduce the extent of the spatial region exposed to theablation beam.

FIG. 12C shows a further alternative method of FSR adjustment by addingmaterial to the laser cavity. Methods that deposit material include thepreferred evaporation process, and also laser induced chemical reactionsometimes called laser pantography, sputtering, and others. In thesputtering or evaporation approaches, the incoming beam 1284 that isdirected onto the surface is a beam of particles, and the directingapparatus 1285 may be an electromagnetic director or a mask or bafflesystem. These methods can use standard equipment to deposit materialuniformly at a controlled rate so that all of the free spectral rangesof all of the resonators on an entire wafer may be shiftedsimultaneously. These approaches proceed in a vacuum, and avacuum-compatible FSR measurement approach is needed such as fibercoupling to external measurement apparatus. In the laser inducedselective deposition technique, the incoming beam 1284 is a laser beam,and the directing apparatus is a focusing optical system that may or maynot include a mask. The laser beam excites material at or near thedeposition surface, inducing a chemical reaction near the region wherethe device is illuminated by the laser, and resulting in deposition.

Again, the spatial location of the portion of intracavity waveguidewhere the index modification is applied may be changed during theprocess. In the deposition method, the rate of change of index decreaseswith time until the location is changed to a fresh portion of theintracavity waveguide when the rate increases again. If the methodchosen is to expose the material to radiation, the index change maysaturate with time at a given location, and changing the locationincreases the rate of change of index at a constant exposure condition.

By illuminating the individual devices on a wafer separately by one ofthese deposition techniques, individual correction of the device FSRsmay be achieved. Because of process variations, FSRs are likely to varyacross a given wafer, and a spatially selective approach such as UVlaser exposure may be preferable.

Frequency Selective Cross Connect

FIG. 13 shows a frequency selective cross connect device as anotherembodiment of the present invention that uses an athermal resonatorconfiguration adjusted to coincide with predetermined frequencies,coupled to a waveguide via a tunable grating. The device 1300 mayconnect light from an input port to alternate output ports, depending onthe frequency of the light. It may be used as a wavelength switch, as anadd-drop device, as a detector, etc.

Waveguides 1322, 1324, and 1326 are fabricated on substrate 1370 fromunder cladding material 1344, core material 1340, and over claddingmaterial 1342 as described elsewhere. Grating 1330 is fabricated in theparallel coupler region 1392 to form preferably a reflective coupler. Asis well known, the propagation constants of the waveguides 1324 and 1322are preferably different, and the wavenumber of the grating 1330 equalsthe sum of the propagation constants of the waveguides in the reflectivegrating configuration. In the alternative codirectional coupler gratingconfiguration, the difference in the propagation constants is used, andthe directions of the light flow as designated by the arrows 1325, 1306,and 1308 are reversed. The transverse profiles of the optical modespropagating in the waveguides 1324 and 1322 overlap each other in theregion of the grating 1330, producing a coupling k1(λ) at the wavelengthλ which is the fraction (in the absence of feedback around the waveguide1324 which forms a resonator) of the power 1302 input in waveguide 1322that is coupled into the waveguide 1324 in the sense shown by arrow1325.

The waveguide 1324 comprises a closed loop, forming a resonator. Powerin waveguide 1324 flows into the parallel coupler 1394 where thepropagation constants of the waveguides 1324 and 1326 are approximatelyequal within a tolerance, producing a coupling k2(λ) of the 1324waveguide power into the waveguide 1326. The wavelength dependence ofthe coupling k2 is much less than that of the coupling k1 in the absenceof a grating in 1394. Alternatively, a second grating structure could beused in 1394 as described below in reference to FIG. 17. This coupledpower exits waveguide 1326 at 1306. The remainder of the power inwaveguide 1324 follows the closed path of waveguide 1324 back into thecoupling region 1392, is partially reflective coupled back into thewaveguide 1322. The power circulating in the waveguide 1324 is due tothe interferometric sum of all the round trip electric field componentsand the input field of the input beam; the exit power 1304 is theinterferometric sum of the transmitted remainder of the field of 1302and the out coupled field from the resonator. The heater stripe 1350,connected to the two electrode pads 1354 and 1356, allows thermo-optictuning of the grating 1330. A detector may be placed to measure thepower 1306, in which case 1300 functions as a detector or receiver.

The resonator 1324 is preferably designed for a FSR that is a rationalfraction of a communications channel spacing, it is adjusted inmanufacturing both to equal the desired FSR and to adjust thelongitudinal mode positions within tolerances, and it is designed to beathermal so as to provide an absolute frequency reference, independentof the heating of the grating 1330. The regions 1328 and if necessary1329 are regions of modification of the index of refraction of thewaveguide 1324 in order to achieve the desired FSR and longitudinal modefrequencies.

In view of the fact that the interaction between the two waveguides atthe grating is preferably strongly dependent on temperature, theathermal requirement on the resonator presents some problems in itsrealization. In the preferred embodiment, these two seeminglycontradictory requirements are met by designing the resonator waveguide1324 to be athermal along every portion of its length, and by using aunique waveguide design shown in FIGS. 41C or 15. To retain tunabilityof the grating, the closely spaced waveguide 1322 is preferably madestrongly temperature dependent.

FIG. 11B shows the preferred athermal design for the waveguide 1324. Incontrast to the design shown in FIG. 11A where the overlap factor Γ_(c)is large, in the cross section of the device 1110 in FIG. 11B, theoverlap factor Γ_(c) in the polymer layer 1142 is just small enough sothat all of the layers together produce zero net change in opticallength n_(eff)L with temperature

d (L n_(eff))/dT=0=d (LΣ_(i)Γ_(i)n_(i))/dT=Σ_(i)Γ_(i)(Ldn_(i)/dT+n_(i)dL/dT)  (10)

where Γ_(i) is the overlap factor for the ith transverse portion of thewaveguide traversed by the optical energy of the mode, and in this casethe sum is taken at a given longitudinal position, over all thetransverse portions of the waveguide. If for example, three materialsare involved in the waveguide: polymer, cladding silica, and coresilica, the sum is taken over these three regions. The waveguide indexcontrast and dimensions and layer thicknesses and etch depths aredesigned so that the polymer overlap factor Γ_(c) takes on the correctvalue to produce a null in equation (10) within a tolerance. Any one ofthe electrodes in FIG. 11B may be used, but for thermo optic excitationa simple single electrode such as 1120 is preferred as a heater trace.In the preferred embodiment of the grating region 1330, the waveguidecross section is shown in FIG. 14C with the waveguide 1324 being thewaveguide on the right with transverse mode 1481.

The preferred fabrication process for the devices 1100 and 1110 startthe same. In both cases, a lower cladding 1144 of silica is fabricatedon a silicon substrate 1170. A core layer is deposited, and the layersconsolidated, patterned, and etched to form the ridge(s) 1140. In thecase of FIG. 11A, layer 1142 is then spun on top, and the waveguide iscomplete. Because the polymer layer 1142 is close to the axis ofpropagation of the mode and the mode 1180 substantially overlaps thepolymer layer (Γ_(p) is large), this structure is a temperature tunableone with a relatively large change in n_(eff) with temperature. Thestructure of FIG. 11A is suitable for the waveguides 1322, 122, 922,1022, 1024, 1726, and the right hand waveguide of FIG. 15.

In the case of FIG. 11B, however, another layer, preferably of silica1141 and of thickness comparable to the core layer thickness, isdeposited and consolidated after patterning the core material 1140. Toachieve a relatively small overlap factor, the layer 1141 is depositedwith thickness in the range of 40% to 200% or more of the thickness ofthe core layer. Layer 1141 is then etched back by a fraction of itsdepth in the range of 0% to about 90%. The cladding layer 1142 is spunon and cured. Depending on the depth of the layer 1141 and the depth ofthe etch-back, more or less of the mode 1181 is revealed for propagationin the clad layer 1142. By adjusting these parameters, the overlapfactor Γ_(c) may conveniently be adjusted below the value obtained inFIG. 11A. If the deposited layer is thin, in the range from 5% to 40% ofthe core layer thickness, when the upper cladding layer 1142 is spun on,the overlap factor will take on an intermediate value.

To obtain an inherently athermal waveguide, since (dn/dT)_(cladding) maybe more than 20 times larger than (dn/dT)_(silica), the overlap factorcan be reduced to the neighborhood of 0.05 or lower, depending on theexact values of the thermo-optic coefficients. Where the condition (10)is satisfied in the waveguide, any longitudinal segment of it may beheated or cooled without affecting the resonator FSR. Transverse thermalgradients across the structures 11A or 11B are preferably avoidedbecause they will introduce residual changes in the FSR and modepositions.

The waveguide 1322, is preferably designed according to FIG. 11A so asto obtain a large tuning rate with temperature. Exciting the heatingstripe 1350 heats the grating region 1330 including segments of bothwaveguides 1322 and 1324. The effective index of the guide 1322 changesrapidly, tuning the grating wavelength. The effective index of theadjacent guide 1324 remains constant and independent of temperature, sothat the longitudinal modes of the resonator are substantiallyunaffected. To maintain the coupling ratio of the coupling region 1394independent of temperature, the waveguide 1326 is preferably designedthe same way as the waveguide 1324.

The reflection band of the grating 1330 is preferably narrower than thecommunications channel spacing. The power transferred from the waveguide1322 to the waveguide 1324 depends on both the grating assisted couplingfunction k1(λ) and the resonator spectrum. The longitudinal mode of theresonator with the largest coupling value is most strongly excited. Forthe structure 1300 to act as a frequency selective detector, thecoupling function should be strong at the selected frequency(ies) andsmall at the adjacent (or all the other deselected) longitudinal modes.A preferred suppression would be greater than about 30 dB. To accomplishthis suppression, the bandwidth of the grating coupler is preferablynarrow so that when the grating is tuned to reflect at one of thelongitudinal modes, the coupling function is very weak at the otherlongitudinal modes. The grating is also preferably apodized and/orchirped and otherwise engineered to accomplish this objective.

The grating 1330 couples a frequency into the waveguide 1324, but if theresonator formed by 1324 is off resonance, the multiple passes aroundthe resonator add up out of phase and suppress the coupling. As thegrating 1330 is tuned through a resonance of the cavity, the multiplepasses add in phase and the coupling is enhanced. There is significantpower coupling with the resonator 1324 only on the frequencies definedby the resonator, despite analog tuning of the coupling grating. Poweris coupled out of the waveguide 1322 one selected frequency at a time,digitally, with strong suppression at intermediate frequencies. Afraction of the power circulating in the waveguide loop 1324 is coupledinto the waveguide 1326. With a detector coupled to the power 1306, thedevice 1300 becomes a tuned detector that can be switched betweenpreselected channels by controlling the current flowing through theheater 1350. The remainder of the power flows transparently through thewaveguide 1322 and into the output 1304. If the device 1300 is anadd/drop device, the output 1304 is connected to the continuing fiber inthe transmission system, with the non-selected frequencies passingthrough with insertion loss but otherwise essentially unchanged. If thedevice 1300 is a tuned detector, the output 1304 is preferably coupledto an absorber and/or an angled reflector and/or diffractor to ensurethat there is no return signal.

Impedance Matching

If the coupling coefficient from the waveguide 1322 into the resonatorformed by 1324 is properly impedance matched at a specific frequency,all of the power 1302 at that frequency will flow through the resonatorand into the port 1306 with no power leaking through to 1304. When theresonator is excited on-resonance, the electric field amplitude coupledin 1392 out of the waveguide 1324 into waveguide 1322 is exactly out ofphase with the remaining input power from 1302 that transmits directlythrough the grating coupler 1330 in waveguide 1322 into the output 1304.Perfect impedance matching is obtained when the resonator loss and thecoupling strength are adjusted so that the amplitudes of those two wavesare identical, resulting in perfect cancellation and zero power flowinto the output channel 1304. With perfect impedance matching, all ofthe input power from 1302 flows into the resonator and is partitionedbetween the output 1306 and the dissipative losses of the resonator1324. If the resonator is imperfectly impedance matched or if it isexcited off resonance, some of the power input at 1302 at that frequencywill be passed through to the output 1304.

The desired situation is to impedance match the coupler 1392 so that atthe predetermined frequency of the resonator that is selected by thetunable grating coupler, all the input power flows to the output 1306.At all other frequencies not selected by the grating, the input powerflows transparently to the output 1304.

Perfect impedance matching can be designed once the dissipative loss ofthe resonator is known, by choosing the resonant coupling constant incoupler 1392 and the coupling constant of coupler 1394 to meet thestated criterion. For efficient operation, the coupling in 1394 shouldpreferably equal the dissipative losses or exceed them by a factor oftwo or so. This leaves the coupling in the coupler 1394 as the main freevariable that is used to achieve perfect impedance matching. Inmanufacturing, as before, perfect impedance matching will not always beobtained of a large enough fraction of the production. A method fortrimming the impedance matching is needed for increasing the yield. Itis possible to adjust the coupling constant of the parallel couplers1392 and/or 1394 and/or the loss of the resonator waveguide 1324 toachieve perfect impedance matching.

The coupling constant in the couplers depends on both the transversemode overlap between the two waveguides in the coupler, and on thestrength of the grating. In trimming the couplers, it is convenient tomeasure a crosstalk quantity during fabrication of the grating and toadjust the grating strength to minimize that crosstalk. For example, ifthe selected crosstalk quantity is the power feedthrough on-resonancepower feedthrough to 1304 from 1302, the ratio of these powers may bemeasured and minimized to produce good impedance matching. This step ispreferably done after the waveguide is fully fabricated, i.e. after thecladding layers are deposited. If the grating is formed by exposure toan energetic beam such as visible or ultraviolet light, the exposure maybe adjusted until the crosstalk quantity lies below the desiredthreshold. For example, UV exposure of the Ge-doped silica core can beperformed by illuminating the core through a fully cured polymercladding layer using the interfering beams approach or possibly a phasemask. Or, exposure of a sensitized polymer cladding layer can beaccomplished by the same means. With good impedance matching, the powerat the resonance frequency that leaks through to 1304 can be reduced toa desirably low value such as −30 dB, for instance.

For example, a test beam may be injected into the waveguide 1322providing an input signal 1302, and a detector placed to be sensitive toa portion of the light exiting the waveguide 1322 as output signal 1304.The ratio of these two signals at the frequency of resonance is a typeof crosstalk called the extinction ratio. This may be done before orafter slicing and dicing of the wafer into individual chips. Bymeasuring the fraction of the input signal 1302 that emerges as anoutput signal 1304 while the exposure of the grating 1330 is proceeding,the total exposure required can be predicted, and the exposure stoppedbefore reaching an over-exposed condition. By remeasuring the extinctionratio, a small increment of exposure may be calculated and applied tonudge the impedance match to the desired optimum. As an alternative, theadd-drop crosstalk may be measured at a resonant frequency as the ratiobetween the output into channel 1306 when the input 1308 is excited, orvice versa.

The transverse mode overlap between the two waveguides may be adjustedby small amounts by changing the index of refraction of the regionbetween the waveguide cores. Again, UV exposure of either a sensitizedsilica region or of an incompletely cured polymer region between thecores can accomplish this objective. This approach has the advantagethat it may be applied independent of the mechanism chosen to fabricatethe grating.

The resonator loss may be increased by several methods. Provided thatthe coupling into the resonator in the grating 1330 is designed toslightly exceed optimum, the resonator loss may be increased to bringthe device to optimal impedance matching. The resonator loss may beincreased by laser ablation or wet or dry etching over a portion of thewaveguide 1324, for instance, gradually bringing a somewhat roughenedsurface closer to the core of the mode in the resonator and increasingthe optical loss due to factors such as diffraction, scattering, andmode coupling loss. Alternatively, an index of refraction discontinuitymay be created transverse to the waveguide by laser ablation or by UVirradiation of sensitized silica or polymer, creating a reflection ordiffraction loss. A shallow angle interface can reflect a large amountof light with even a small index change. Other approaches are alsopossible such as by doping a photo chromic molecule into the resonatorto produce changes in absorption upon exposure. Or, a surface absorptionloss may be introduced for example by depositing a metallic film nearenough to the waveguide core to produce some optical loss. The lossinduced by this film may be controllably trimmed away by e.g. laserablating portions of the film, thereby also adjusting the resonatorloss.

Note the desirable fact that if the grating drifts slightly in centerwavelength, the transmission spectrum of the output beam 1306 is littlechanged. The effect of drift shows up as a change in the side modesuppression, and as a reduction in efficiency of power transfer to theoutput leg 1306 (reducing the detection efficiency of a detector mountedto receive the power 1306).

When the cavity is athermal, the grating coupler is tunable, the FSR andone of the modes has been frequency adjusted, and the impedance matchingis complete, the device 1300 may be used as a digital wavelengthselective cross connect. A multiple frequency (DWDM) input may beconnected as 1302, and selected ones of the frequency channels may beredirected from 1304 to 1306 with good efficiency and low crosstalk. Thedevice 1300 is essentially transparent at the other frequencies.

An additional input port allows power to enter the system at 1308. Bysymmetry and time reversal invariance, if the coupler 1392 is impedancematched at a given frequency from the input 1302 into the output 1306,then the coupler 1394 will also be impedance matched at the samefrequency from the input 1308 into the output 1304. This means that whenthe device is tuned to efficiently drop a channel into 1306, it is alsotuned to efficiently add a channel from 1308 to 1304. Data at theoptical frequency of coupling determined by the grating 1330 and itsactuator 1350 may therefore be dropped out of 1302 and added into 1304,substantially independently if the crosstalk is small, and withoutaffecting the other frequency channels of the system.

The waveguide 1324 forms a resonator. Light makes multiple round tripsbefore decaying away due to losses and coupling out of the resonator.The multiple round trips interfere with each other, producing thecharacteristic FSR and longitudinal mode spectrum already referred to,along with the phenomena of impedance matching. This situation is inmarked contrast with the situation in which a totally reflective gratingis placed in the coupler region 1392. In the high reflector gratingcase, light 1302 enters the waveguide 1324 after one reflection from thegrating 1330, makes a single pass around the loop 1324, and exits aslight 1304, without substantial interference from multiple round tripsin the loop 1324. The functioning of such a high reflective gratingdevice would be essentially the same whether or not the waveguide 1324forms a resonator. This high reflector grating case is described in A.S. Kewitsch, U.S. Pat. No. 5,875,272, Feb. 23, 1999. Withoutinterference, the useful longitudinal mode resonances disappear.

The waveguides 1322 or 1326 may be coupled to other devices, includingfibers, amplifiers, switches, reflectors, filters, modulators, sources,and detectors. In particular, 1326 may also be a communicationswaveguide carrying multiple optical frequency channels that enter at1308. In the configuration of FIG. 13, the resonator 1324 is coupledsimultaneously to substantially all these wavelengths since the coupler1394 is a broadband coupler. Light input into 1308 with the frequencyselected by the grating 1330 will be transferred into the output 1304.In this configuration, the device 1300 acts as a channel-selectivecross-connect switch, with only the selected frequency (or frequencies,depending on the grating design) being switched from 1302 to 1306 andfrom 1308 to 1304, and all the other frequencies passing through.However, as shown, the waveguide 1326 is not as transparent as thewaveguide 1322 because the coupler 1394 is broadband, and couples manyfrequencies into the resonator 1324, whereas the coupler 1392 isnarrowband and may couple only one frequency into the resonator.Although the frequencies that are coupled from waveguide 1326 into theresonator may not be coupled into the waveguide 1322 if the grating isoff resonance, there will still be distortion in 1326 at the couplingfrequencies, such as pulse distortion and additional power loss.

FIG. 17 shows another embodiment 1700 of the invention in which thecoupler 1794 is also fabricated as a narrowband coupler comprisingelements similar to those of coupler 1392. A thermo-optic grating 1730and heater stripe 1750 are provided as described. The waveguides 1324and 1726 have dissimilar mode propagation constants, and preferablydissimilar thermal coefficients as described in relation to FIG. 14C or15. Waveguide 1726 is fabricated in the same way as waveguide 1322. Theinput 1708 and output 1706 have reversed position from FIG. 13 since thegrating coupler in 1794 is reflective. The functioning of the device1700 may be almost identical to that of the device 1300 except that thewaveguide 1726 is now transparent at the frequencies away from thegrating resonance(s). The inputs 1708 and 1302 may be symmetric, as forthe outputs 1706 and 1304. To couple a specific frequency across theresonator from 1302 to 1706 or from 1708 to 1304, both gratings 1330 and1730 are typically tuned to the desired frequency. Multiple opticalfrequency channels may be present on both 1302 and 1708, and the data ona specified channel is interchanged when both gratings are tuned to thatfrequency. The device 1700 therefore may act as a frequency selectivecross connect or optical switch, including the subset functions ofadd-drop and tuned detector. The two gratings 1730 and 1330 may be tunedsimultaneously by the same heater current, but there are advantages tobeing able to tune them separately.

By cascading multiple devices such as 1700 or 1300 along a single inputchannel 1302 (connecting 1304[i] to 1302[i+1]) multiple optical channelsmay be independently dropped into multiple output channels 1706[i] oradded from multiple input channels 1708[i]. By cascading multipledevices such as 1700 or 1300 along two input channels (connecting1304[i] to 1302[i+1] and connecting 1708[i] to 1706[i+1]), multipleoptical channels may be independently cross connected between the twofibers each carrying multiple data channels.

The devices 1300 and 1700 are reconfigurable since they may be changedfrom acting on (i.e. cross-connecting) channel i to acting on channel j.It may be desirable, when tuning the device 1700 from channel i tochannel j, to avoid acting on the intermediate channels that lie betweenchannels i and j. If channels i and j are separated by activecommunications channels carrying data, it is essential to avoidsignificantly perturbing or “hitting” the data transmission in thoseactive channels when tuning a device from cross-connecting channel i tocross-connecting channel j. The device 1700 offers a unique way to avoidhitting the intermediate channels. The two gratings 1330 and 1730 may betuned at different times or different rates so that their frequencies donot overlap at the intermediate channels but only at the desired endchannels. If the two grating responses do not overlap, impedancematching is not obtained across the resonator, and the interaction withan intermediate channel while tuning across it is greatly diminished: nolight will be added, dropped, or switched. (There will be a perturbationdue to the coupling into the resonator 1324, but if the tuning isaccomplished slowly, this perturbation may be negligible.) To tune fromchannel i to channel j, for instance, one grating may be tuned first,and then the other. The action on channel i will cease as soon as thefirst grating is detuned, and the action on channel j will not beginuntil both the first and the second gratings are tuned to channel j. Orin another example, the two gratings may be detuned relative to oneanother by a desired amount to stop the action, after which the detunedpair is tuned across to the desired channel where they are retuned intocoincidence with each other.

FIG. 18 illustrates a vernier method of extending the tuning range byusing multiple grating peaks. The longitudinal modes of the resonator1324 are shown as a function of frequency by the multipeaked curve 1810where the width of the resonances has been shown to be very small forsimplicity. We assume that any birefringence has been compensated sothat the curves for the two polarizations overlap. The resonator FSR1814 is the separation between adjacent peaks of the curve 1810. The twogratings 1330 and 1730 each have multiple peaks, as shown by the curves1820 and 1830, respectively, which each have five peaks. The peaks ofthe gratings are essentially equally spaced, but the peak spacing in onegrating may be larger than that of the other grating by the frequency1812, which in FIG. 18 is equal to twice the FSR. For the tuning shownin FIG. 18, a pair of grating peaks 1823 and 1833 coincide in frequency.By tuning the pair of gratings together over the range 1840 (7 FSR),eight adjacent longitudinal modes 1852 may be selected. If the FSRequals one communications channel separation and the modes 1810 overlapthe channels as described, eight successive communications channels maybe selected by the device 1700.

When one spectrum e.g. 1830 is detuned relative to the other by acertain amount, in this case by one FSR, none of the modes overlap, andno mode is active in the device 1700. The frequencies of the two spectramay be tuned together in this condition without dropping or switchingany of the channels. By tuning the spectrum 1830 of the grating 1730 bytwo FSR increments to lower frequency, the second pair of grating peaks1824 and 1834 may be brought into coincidence. The next eight successivechannels 1854 may be selected by again tuning the two gratings togetherover the range 1840, relative to the starting point where the gratingpeak 1834 has been adjusted to overlap 1824. It apparent clear that bytuning the spectrum 1830 by either 0, ±2 FSR, or ±4 FSR, and by tuningthe pair of spectra 1820 and 1830 together over the range 1840, a totalof at least 5×8=40 sequential channels 1850 may be addressed. Therequired tuning is only about 16 FSR, including the initial tuningrequired to superpose the grating peaks 1823 and 1833 on the desiredchannel. The number of channels that can be addressed for a given tuningrange has been increased by the use of this vernier method at theexpense of the additional channel perturbation caused by the additionalgrating peaks, which is preferably made small.

If it is desired to jump between noncontiguous tuning ranges, such asfrom the range 1852 to the range 1856, the intermediate modes such as1824 and 1834 must be made to overlap somewhere. If intermediatechannels are in use for data transmission for example, so that theycannot be hit, the tuning may be adjusted so that the modes 1824 and1834 overlap at intermediate frequencies such as in between longitudinalmodes 1810. Provided that the grating spectra of the peaks 1824 and 1834are narrow, the coupling by the gratings into or out of the resonator1324 can be made small.

Many variations of this approach may be used. If the grating peaks arenarrow enough, the frequency interval 1812 may be smaller than two FSRs,allowing use of additional grating peaks. If the tuning range coversmore (or less) than eight channels, the tuning range may also beincreased (or decreased). Unequally spaced peaks may be used, etc. As afurther alternative, the device 1300 or 1700 may be combined with a pairof optical switches on either side and a bypass waveguide that route anoptical input either through the device or around it along the bypasswaveguide to an optical output in an arrangement known in the art as ahitless switch, so that the optical transmission may be switched tobypass the device during the tuning of the grating 1330.

Differential Waveguide Thermal Response

FIGS. 14A and 14B are two intermediate stages in the fabrication of theembodiment of FIG. 14C which contains a pair of adjacent waveguidesfabricated with different (preferably thermo-optic) coefficients. Thefirst steps of fabrication of such a structure preferably involve thedeposition of a cladding layer 1444 followed by a core layer with higherindex of refraction on a substrate 1470. As described previously, thisstructure is preferably fabricated from silica on a silicon wafer, withGe doping for the core, but many variations are available. The corelayer may then be patterned, producing the twin parallel ridgestructures 1440, seen in cross section end-on. The top cladding 1441 isthen applied and an etch mask layer 1443 is deposited and patterned toreveal only one of the two waveguides along a desired portion of itslength. FIG. 14A shows the semi-processed article at this stage in itsfabrication. An etching step such as RIE is now applied to remove someof the silica cladding in the regions not covered by the mask, resultingin the structure of FIG. 4B. After removing the mask material of layer1443, a thick polymer cladding layer 1442 is spun on, and anelectrically conductive layer may be deposited and patterned to form theelectrode structure 1445, resulting in the device of FIG. 14C.Additional layers of electrodes or additional waveguide cores, gratings,and other components may optionally be fabricated either on top of, in,or below this structure.

In the case of the grating coupler 1392 or 1794 for instance, it isdesirable to fabricate a grating such as grating 1330 in the layer 1441between the two core regions 1440 (or in another portion of thestructure traversed by optical modes 1482 and 1481). This grating may befabricated at several alternative times during an overall processsequence, including before depositing the layer 1443, and after applyingthe layer 1442. It is apparent from FIG. 14C that the mode 1482 has amuch larger overlap factor in the layer 1442 than the mode 1481. Ifdesired, the waveguide design and the layer characteristics includingthickness may be adjusted so that the mode 1481 has no net temperaturedependence of its effective index of refraction (all such conditions aremet within a tolerance). Then the mode 1482 of the adjacent waveguidehas a net negative thermal tunability while its partner is athermal, ascalled for in the preferred design of the devices 1300 and 1700. Ifdesired the temperature dependence of the two waveguides may also berealized as equal and opposite by further reducing the overlap factor ofthe modes in the negative thermo-optic coefficient material 1442. Othervariations are also possible including making the mode 1482 athermal, inwhich case the mode 1481 has a positive thermal coefficient.

In a different embodiment, the layer 1442 is an electro-optic, and theelement 1445 is an electric field applying electrode. In this case thechange in effective index with applied field is different for the twowaveguides 1440 and for the two modes 1481 and 1482. As before, anelectrode configuration similar to that of FIG. 11B may be preferred forelectro-optic devices.

FIG. 15 shows an alternative embodiment of an adjacent pair ofwaveguides with different coefficients. The fabrication techniqueinvolves a variation of the fabrication technique described in referenceto FIGS. 11A and 11B, but with the addition of a masking step, an etchstep, and another deposition step. An adjacent pair of waveguide coresare fabricated as described e.g. with reference to FIG. 11A. A claddinglayer 1544 and a core layer 1540 are deposited on a substrate 1570. Thecore layer is patterned to form the twin waveguide cores shown in FIG.15, and cladding layer 1542 is deposited. A mask layer may be appliedabove the layer 1542 and patterned in the same way as described inreference to the mask 1443 in FIG. 14A, to reveal only one of the twowaveguide cores along a portion of its length. An etch process isapplied to remove the layer 1542 where it is exposed by the mask asshown on the right side of FIG. 15. This step is preferably a selectiveetch so that all of the layer 1542 is removed where exposed, withoutsubstantial removal of the silica layers 1544 or 1540. A final layer1546 is applied, producing the structure of FIG. 15. Optionally, thesurface may then be planarized. In the case of FIG. 15, the modes thatpropagate in the two different cores may have different thermalcoefficients because of different thermal characteristics of materialsused for the layers 1542 and 1546. If, for example, a heavily crosslinked polymer is used for layer 1542, and a lightly cross linked layerof a similar polymer is used for layer 1546, all other things beingequal, the mode with the larger overlap factor in the more lightly crosslinked material 1546 will have the stronger thermal coefficient(assuming the thermal coefficient is dominated by the layers 1542 and1546). Or, if the operating temperature is well below the glasstransition temperature Tg of the polymer 1542, but well above the Tg ofthe polymer 1546, the mode with the larger overlap factor in theabove-Tg material 1546 will have the larger (negative) thermalcoefficient. As an alternative in any of the above, if a layer such as1542 is itself photosensitive, an additional masking layer may beomitted, and the layer such as 1542 may be patterned directly byexposure and development to form the desired patterned structure.

In the implementations of the invention described herein, a liquidcrystal material may be used to change the index of refraction of awaveguide. For instance, layer 1546, or 1442, or 1142, or 842, or 742,or 342 or other layers traversed by the optical mode within the gratingregion may be a liquid crystal material of one of the many types knownin the art. A particularly convenient case is that of the polymerdispersed liquid crystal (PDLC), which may be considered anelectro-optic material since the index of refraction changes withapplied field (although the response time is relatively slow compared toelectro-optic crystals, for example). Take for example the case of FIG.11B where layer 1142 may be a PDLC. Electrodes 1122, 1124, and 1126 areprovided to produce electric fields with controllable vertical andhorizontal components. An upper cladding (not shown) may also be used toincrease the voltage breakdown threshold between electrodes on the samesurface such as 1122 and 1124. When electrodes 1122 and 1124 are excitedto the same polarity different from 1126, a substantially vertical fieldis produced in the vicinity of the waveguide, lining up the liquidcrystal material in the PDLC in the vertical direction. When electrodes1122 and 1124 are excited to opposite potentials and 1126 is at groundpotential, a substantially horizontal field is produced in the vicinityof the waveguide core, lining up the liquid crystal material in the PDLCin the horizontal direction. The index of refraction of the PDLC and theeffective index of the waveguide is substantially different in the twocases for any component of polarized light (either TE or TM) in thewaveguide. The index of refraction may be varied continuously bychanging the potentials to the electrodes appropriately between the twoextreme situations described above. If a grating has been fabricatedalong the section of the waveguide with PDLC (such as in the corematerial 1140, or in one of the cladding materials 1144, 1141, or 1142within the optical mode 1181), the interaction of that grating will betuned by applying fields as described. Note that the waveguide structureof FIG. 11A is preferably used to obtain a large tuning range, but theelectrode structure of FIG. 11B is preferably used to tune the PDLC. Thepolymer matrix of the PDLC is also preferably selected to approximatelyindex match with the liquid crystal materials near an operatingcondition, to reduce losses.

A PDLC is a convenient type of liquid crystal material to use in thisapplication because it does not require confinement, the confinement ofthe liquid crystal droplets being by the polymer of the PDLC. However,PDLC has the disadvantage of relatively large optical scattering,depending on the indices of refraction of the aligned liquid crystaldroplets and the polymer matrix. More standard liquid crystal materialsmay also be used. With these latter materials, a confinement structureis preferably provided. The electrodes 1120, or if needed electrodes1122 and 1124 may be provided on the top plate or on the additionallayers, or otherwise disposed spatially to create the desired electricfields. Alignment layers for the liquid crystal materials and electrodescan be provided to actuate the liquid crystal material as is known inthe art. Electrodes may alternatively be provided longitudinallyarranged along the waveguide with the electric field parallel to theguide. Transparent electrodes as known in the art may be used todiminish optical losses if a significant portion of the optical modetraverses the electrode material. Excitation of the electrodes changethe index of refraction of the liquid crystal material, either toincrease or decrease the index of refraction, depending on thearrangement of the initial voltage and/or the alignment layer thatestablishes molecular alignment in the absence of an applied field.

The athermal resonators described herein provide a useful function inthemselves, even without any tuning function. For such devices, theheater stripes may be omitted, and the grating elements may befabricated in athermal waveguides to accomplish temperature independentfrequency selective devices. For example, the devices of FIGS. 1, 9, and10, may be used as fixed frequency laser sources, and the devices ofFIGS. 13 and 17 may be used as fixed-frequency cross connects or asfixed frequency detectors or add-drop devices.

In another alternative embodiment, all of the elements of the structureare integrated onto a single substrate. If the substrate is InP, thelaser structure can operate in either the 1310 nm region or the 1550 nmregion; other substrate materials enable other wavelengths. In allcases, a polymer material may be integrated into a portion of theoptical cavity along the waveguide, and a frequency selective structuremay be used to determine the operating frequency. The polymer structuremay be tuned to produce an athermal free spectral range, the FSR may beadjusted to equal a rational fraction of a communications channelspacing, and the grating may be tuned by a polymer section.

Tapered Coupling

FIG. 3 shows the preferred embodiment of a hybrid integrated couplingapparatus including a taper, but ignoring other complexities such as thewaveguide bends of FIG. 5. A simple taper described for example in * HG. Unger, U.S. Pat. No. 4,415,227, Nov. 15, 1983 is often insufficientfor coupling single mode waveguides with largely different modedimensions. The normalized propagation constant V of a waveguide helpsdetermine its modal behavior:

 V=k_(o)T(n_(f) ²−n_(s) ²),  (11)

where k_(o)=2π/λ, T is approximately equal to the smallest transversedimension of the waveguide, n_(f) is the index of refraction of the corelayer of that waveguide, and n_(s) is approximately the index ofrefraction of the underlying layer adjacent to the waveguide. If forexample one of the waveguides operates in the middle of the single moderegion with a normalized propagation constant V=2.5, a factor of twoup-taper will result in multimode behavior. On the other hand, a downtaper of such a waveguide is not effective in reducing the modedimensions. Our problem is to match a small, high-contrast single modewaveguide in a semiconductor diode laser to a larger, low contrastsingle mode waveguide in a planar waveguide chip. We wish to fabricatethe taper on the more easily manufactured passive waveguide substrate.We have designed a three-waveguide approach where we use a thirdmaterial of index of refraction intermediate between the indices ofeither the small or the larger waveguides, and we use an extreme taperthat drives the third waveguide below cutoff for the operatingwavelength.

The cutoff of a waveguide is defined as the parameter value of interest(e.g. wavelength, width, effective index, etc.) past which thetransverse confinement of the mode is lost and energy propagates awayfrom the core transverse of the propagation axis of the guide. Thecutoff situation is to be distinguished from the coupling between welldefined waveguides where energy is exchanged between well defined(otherwise below cutoff) waveguides.

Diode laser chip 310 contains a waveguide section 320 fabricated on asubstrate 314. The laser chip 310 provides net amplification around arange of wavelengths such as 1552 nm. The diode laser chip 310 hasdimensions that are selected according to the wavelength and materialsystem and might be 500 microns thick, 400 microns long in the directionof the waveguide, and 400 microns wide, for a 1550 nm laser. The rearfacet 312 forms one end of the laser cavity. The laser lases along theoptical axis 324. The other end of the laser cavity may be inside thewaveguide 340 at the location of a retroreflecting grating (see 130 or132 of FIG. 1). The dimensions of the waveguide section 320 are variablebut may typically be about 0.7 microns in the y direction, and about 2.1microns in the x direction, with an optical mode size of perhaps 0.55microns and 1.65 microns, respectively. We typically quote the mode sizeas the 1/e² half width of the intensity. The full width of the mode isabout twice the mode size, and the full width at half maximum of thebeam is about 1.18 times the mode size. The laser propagation axis 324along the waveguide 320 is fixed at a vertical distance 325 above thelaser substrate 314, in preparation for bonding and alignment onto thesubstrate 370.

The laser is preferably attached to the substrate with solder as shownin FIG. 3. The laser lower surface is prepared with an adhesion layer352, a barrier layer 354, and a cap layer 356. Note that the laser chipis upside-down in FIG. 3 since the substrate 314 is to the top of thefigure. The adhesion layer is preferably 0.1 microns of Ti, the barrierlayer 0.4 microns of Pt, and the cap layer 0.1 microns of Au. Thebonding surface of the substrate 370 may be prepared in a similar waywith an adhesion layer 358, a barrier layer 360, and a cap 362. Thesolder layer 350 is prepared on the cap layer of the substrate forconvenience, and may be fabricated from a few microns of AuSn solderwith a melting point of about 280° C. The laser waveguide may be alignedin the x-z plane, pressed down into place, and the solder heated for ashort time in an appropriate gas such as formic acid to prevent theformation of oxides and to allow the laser chip to bond. Notice that thethicknesses of the solder layer 350 and the other layers are controlledso that the laser substrate 314 after bonding has a desired verticalseparation 326 from the waveguide substrate 370, within a tolerance forgood optical coupling. Since the laser waveguide 320 lies a fixeddistance above the laser substrate 314 according to the process sequencein the fabrication of the laser, and the waveguide 330 lies a fixeddistance 335 above the waveguide substrate 370 according to the processsequence in the fabrication of the waveguide 330, it follows that bycontrolling the bonding layers 350, 352, 354, 356, 358, 360, and 362 andthe bonding process, we can adjust and control the relative verticalpositions of the waveguides 320 and 330 by adjusting and controlling theseparation between the two substrates 314 and 370. Within the verticaltolerance, the vertical separation 326 may be adjusted to equal the sumof the vertical height 325 of the waveguide 320 and the vertical height335 of the waveguide 330.

The waveguide 340 fabricated has dimensions chosen to optimize otherfactors including the efficiency of coupling to standard optical fiber,the fraction of the mode that propagates in the cladding, and thepropagation loss through structures including bends. While again thedimensions of this waveguide may vary by a factor of three to ten or so,depending on these factors and fabrication factors such as indexdifference, we have chosen commercial silica technology for waveguide340 with a preference for a waveguide dimension of 2 microns by 2microns with a 2% index difference core-to-clad.

The disparity in the dimensions of the two waveguides 320 and 340 wouldresult in a high coupling loss if waveguide 320 were to be butt coupleddirectly to waveguide 340 (close to 4 dB), even with perfect alignment.In the hybrid grating laser, this loss is intracavity and will increasethe threshold and reduce the output power. The tapered waveguide section330 is preferably provided to improve the coupling efficiency to betterthan 3 dB and preferably less than 1 dB or even 0.5 dB. Waveguide 330 ispreferably butt coupled to waveguide 320, and parallel-coupled towaveguide 340 with a special taper that drops the effective index of thetapered waveguide 330 below cutoff. Ideally, the input end 332 of thewaveguide 330 is designed so that the transverse dimensions of thelowest order optical mode propagating in the input segment of 330 areequal to or near the transverse dimensions of the lowest order opticalmode emerging from the diode laser waveguide 320. By providing an indexmatching material in the gap between the two waveguides, the Fresnelreflections and optical scatter from the two waveguide ends can also beminimized. FIG. 3 shows that the waveguide 330 is preferably fabricateddirectly on top of the waveguide 340, but in an alternate design may bebelow it or separated by additional layers.

The tapered portion of the waveguide 330 brings the propagation constantof waveguide 330 close to that of the waveguide 340 allowing energy tocouple between the waveguides. The taper continues until the waveguide330 goes below cutoff. Light propagating out of the laser waveguide 320is butt coupled into the end of the tapered waveguide 330; in thedown-tapered region it is in turn transversely coupled into the parallelwaveguide 340. Light returning from the opposite direction in waveguide340 is transversely coupled into the same portion of the waveguide 330which is now up-tapered for light propagating towards the laser,whereupon it is butt coupled into the waveguide 320 of the diode laserchip. The waveguide 340 is fabricated from a material with index ofrefraction n₂. The surrounding materials have lower indices close to andpossibly identical to that of silica n₁. The strength Δ of the waveguide340 depends on the local transverse dimensions, the index of thecladding 342 if different from that of silica, and the index difference

Δ₃₄₀=(n₂−n₁)/n₁.  (12)

Depending on these parameters, the waveguide 340 will guide a lowestorder optical mode that has an effective index of refraction n_(eff)that lies approximately in the range n₁<n_(eff)<n₂.

FIG. 4 illustrates the changes in index of refraction produced by theoperation of the taper. For the waveguide 330 to function, its index ofrefraction n₃ is larger than n₂. As shown in the region 460 in FIG. 4,the n_(eff) 430 of the first segment of the waveguide 330 is larger thanthe n_(eff) 440 of the first segment of the waveguide 340. A taperreduces the strength of the waveguide over the region 470, lowering then_(eff) 430 and changing the mode shape. The taper can be accomplishedby varying the local transverse dimensions and the index difference, butit is easier to vary the lateral width of the waveguide by changing themask used to determine the waveguide pattern during fabrication. Whilethe waveguide 330 is still relatively strong, the taper can be rapid, asshown in the region along the z axis of extent T₁ in FIGS. 3 and 4.However, as the taper progresses, the mode of the waveguide 330 beginsto couple with the modes of the waveguide 340 in the coupling region450.

Coupling will be strongest with the lowest order mode of the waveguide340 because their effective indices are closest. Energy exchange occursover a limited region 450 in z where the two mode indices approach eachother, and where the respective indices acquire an imaginary part.Looking at the propagation direction of increasing z, as the taperprogresses, the two modes perturb each other more strongly, and thecoupling becomes stronger. Eventually the mode of the waveguide 330disappears (below cutoff), the imaginary part of the index returns tozero, energy exchange ceases, and the perturbation of the lowest ordermode of waveguide 340 disappears. To accomplish an efficient transfer ofenergy from waveguide 330 into waveguide 340, it is desirable toaccomplish the taper slowly while the modes couple, allowing sufficientinteraction time for completion of energy exchange. The region T₂ istypically longer than the region T₁ for this reason.

There are many choices to be made in selecting a particular waveguideand taper design. Materials for the upper and lower waveguides arepreferably stable, easily processable, and compatible. Even within agiven choice of materials set, there remain many options and severaldegrees of freedom. The preferred taper design for this application usesa silica (SiO₂) materials system for the waveguide 340 and lowercladding 344, and a tantala (Ta₂O₅) core for the tapered waveguide 330,with a polymer top cladding 342.

For the upper waveguide 330 we choose tantala because of its stability,low loss, and high index n₃=2.03 at 1.55 microns. Note that the exactindex of refraction depends on the process used to deposit the film.Since the taper characteristics depend sensitively on the index, all ofthe numbers quoted here are calculated for n₃=2.03, but are preferablyadjusted experimentally to obtain the desired performance for a givenmanufacturing process. The vertical dimension of the input end 332 ofthe waveguide 330 is preferably chosen to be about 0.08 micron (in the ydimension), producing a very weak vertical guide with a vertical modesize of about 0.8 microns that approximately matches the verticaldimension of the guided mode in the diode laser. The lateral dimensionis preferably about 2 microns (in the x direction), producing a lateralmode size of about 1.3 microns, close enough to the diode laser modesize of about 1.65 microns. The effective index of this guide ispreferably just slightly above cutoff, at approximately n_(eff)=1.49, sothat the mode full width is substantially larger than the coredimensions, particularly in the vertical direction. The length of theinitial taper may be short, in the region of T₁=100 microns or so, andthe majority of the taper may be accomplished here, reducing the widthof the waveguide by a factor that may be 2 or even 10 or more, dependingon the specific design. The exact length and amount of taper in thisregion can be adjusted experimentally to optimize the amount of modesize reduction while minimizing the optical loss (the length T₁ can beincreased to reduce the optical loss). Most of the taper but essentiallyno coupling is preferably accomplished in this rapid taper section. Thesecond taper may be longer such as T₂=500 microns or so, and sufficienttaper is provided to initiate the coupling between the modes, to carrythe upper waveguide mode below cutoff, and to decouple and substantiallyreduce the perturbation of the lower waveguide. It may be sufficient totaper this portion of the guide by 50%. Again, the exact length andamount of taper in this region may be adjusted experimentally tooptimize the amount of energy transfer between modes (the length T₂ canbe increased to reduce the optical loss). The initial untapered portionmay be very short or even of zero length, or multiple tapered sectionsmay be provided.

The preferred taper design uses a 0.08 micron tantala thickness and asingle taper (i.e. T₁=0) in which the lateral waveguide width is tapereddown from 2 microns to 0.7 microns over a distance T₂=600 microns. Asthe mask width becomes smaller than about 1 micron, factors such aslithographic resolution and mask undercutting make it increasinglydifficult to reproduce small features. Although the mask provides taperin the lateral dimension, the process of reproducing the mask anddefining the waveguide width will provide both somewhat narrower lateralfeatures than 0.7 microns, and a natural vertical taper that is morepronounced at the small end of the waveguide 330. For this reason, theeffects of the fabrication equipment on the function of the taper shouldpreferably be taken into account, and the dimensions adjustedaccordingly. Indeed, this fact may be made use of in the design torealize a lower effective index at the thin end of the waveguide than iscalled for in the mask design. At the end of the taper, the waveguide330 may be terminated abruptly because the waveguide 330 is below cutoffand very little optical energy remains in the region of the core.Alternatively, the mask taper may be continued to zero width, allowingthe lithographic process to terminate the waveguide naturally.

FIG. 8 shows an alternative taper design in which segmentation isincluded in the design of the waveguide. A substrate 870 is preparedwith a waveguide core 840 of index n₂, upper cladding 842 of indexapproximately n₁, and lower cladding 844 of index n₁. The waveguide core840 has higher index of refraction than the cladding layers so that itguides at least one optical mode. Segmented waveguide 830 is fabricatedon the waveguide 840 with a layer of a desired thickness and index n₃,it is patterned by removing undesired material into a tapered width thatis preferably wider laterally at the interface 832, and into a segmentedpattern by removing material in the regions 838. The removal of materialis shown as complete, but the removal may extend down to some degreeinto the core material of waveguide 840, or the removal may beincomplete, leaving some material behind, bridging the segments 836 (notshown). Notice that the optical propagation axis 834 is a determineddistance 835 above the substrate 870, in preparation for a second chipto be attached on the empty portion 872 of the substrate as discussedelsewhere and in relation to FIG. 3. As a variation, the waveguide 840may be fabricated on waveguide 830, in which case, the segments 836 willbe embedded into the core material of 840 instead of into the claddingmaterial of 842. The device 800 may be a portion of the devices 300 or100 or other devices.

As shown, in FIG. 8, the high index waveguide 830 is fabricated fromregions 836 which are interspersed with regions 838 (segments) where thehigh index material has been removed. Provided that the segmentationperiod (the sum of the widths of a region 836 and an adjacent region 838along the direction of optical propagation) is comparable or less thanthe vertical and lateral Rayleigh ranges

z_(o)=πw_(o) ²/λ  (13)

where z_(o) is the Rayleigh range, w_(o) is the 1/e² optical beamintensity half width, and λ is the vacuum optical wavelength of themode, the optical mode will propagate as if the waveguide core werecontinuous (non-segmented) but had a lower index than that of theregions 836. The effective index contrast with the cladding 842 isreduced by the local duty factor

DF_(seg)=(local length of segment)/(local segmentation period)  (14)

of the segmentation. A duty factor of 50%, obtained when the segmentsand the removed regions are of equal length, will reduce the effectiveindex of the waveguide by a factor of approximately two. By adjustingthe parameters of the waveguide appropriately, the desired mode sizescan be obtained with a segmented guide, but additional degrees offreedom are now available to the designer: the duty factor and theperiod of the segmentation. For example, if the duty factor is 50% inthe region near the input of the waveguide, and the thickness of thetantala film used to fabricate the segments 836 is increased by a factorof 1.414 compared to the previous description to 0.11 microns, thewaveguide strength and mode sizes in the vertical and transversedimensions will be approximately the same as described above.

The duty factor may be reduced along the propagation axis 834 of thewaveguide 830 by changing the lithographic mask pattern used infabricating the segmented waveguide 830, aiding in the accomplishment ofthe taper. The taper required for the non-segmented waveguide describedabove has a waveguide width at the small end of the waveguide of 0.7microns. Such a small dimension may be a challenge to fabricatereproducibly. Larger minimum dimensions are preferred. With the use ofsegmentation, we choose a minimum dimension of 1.0 micron to obtain thesame optical effect of the taper, in terms of the variation of theeffective index of the optical mode. The waveguide 830 now taperslaterally from a 2 micron width to a 1 micron width at the small end,the length of the segments 836 is kept constant at 1 micron, and theduty factor is varied from 50% to 25% by increasing the length of theremoval regions 838 gradually from 1 micron to 4 microns at the smallend. Note that many variations of the functional form of the taper ofthe segmentation are possible, and many others can be useful, includinga linear taper of the duty factor, exponential, hyperbolic, sinusoidal,and all the other mathematical forms. Note also that the generalsegmentation taper includes the possibility of a taper in the periodwith alternative functional forms, provided that the period continues toobey the constraint of being comparable or less than the Rayleighranges. Many variations are also available in the geometry ofsegmentation, such as the indented geometry of R. Adar, U.S. Pat. No.5,577,141, Nov. 19, 1996, “Two-dimensional segmentation mode taperingfor integrated optic waveguides”, multiple superposed layers ofvariously segmented materials, etc.

The tantala waveguides shown in FIGS. 1, 3, 5, and 8 are preferablyfabricated by deposition of a tantala film of the desired thicknessafter the fabrication of the germania/silica core. The tantala film maythen be patterned by RIE after a masking step to define the desiredboundaries of the tantala waveguide segments. Several variations areavailable, but not shown in the figures, including depositing thetantala waveguide after the top cladding has been deposited. In thelatter case, the top cladding is patterned and removed down close to thetop surface of the germania/silica core wherever the tantala waveguidestructure is desired; a uniform deposition of the desired thickness oftantala is then sufficient to create the desired waveguide.

Lensed Waveguide End

FIG. 7 shows an alternative preferred taper embodiment that can providegood coupling efficiency between two dissimilar waveguides. Thisapproach is based on lensing the waveguide end. FIG. 7A shows asubstrate 770 prepared with a waveguide core 740 upper cladding 742 andlower cladding 744. The waveguide core 740 has higher index ofrefraction than the cladding layers so that it guides at least oneoptical mode. As with the waveguides described elsewhere herein, thiswaveguide may be called a single mode waveguide if it guidespredominantly a single mode. The higher order modes may be cut off, inwhich case the guide is strictly single mode, or a few poorly confinedand higher loss modes may be weakly guided, in which case the guide iseffectively single mode which may still be useful for many applicationsincluding coupling to single mode optical fibers. A region 752 of thecore 740 projects slightly from the surface 732. Although the projectionis shown with sharp edges in FIG. 7A, these edges may be rounded.

This projection may be fabricated by applying a two step selective etch.First, the waveguide may be etched vertically by a non selective etchingprocess that etches both the cladding and core layers at similar rates,exposing upper and lower cladding in the region where the core 740emerges at the surface. In this step an etch barrier such as a metalcoating of Au or Cr is deposited onto the surface of the semi-processedarticle. A patterned layer of photo resist is applied on top of the etchbarrier in the desired pattern of the removal region (and other patternsif desired) and etched to transfer the photo resist pattern to the etchbarrier. A reactive ion etching process may be used to etch down intothe silica layers left exposed by the patterned etch barrier layer. Theetch time is preferably controlled to allow an etch depth large enoughto etch through the core 740 and into the lower cladding 744. The etchproceeds approximately vertically down towards the substrate 770. Manyprocess alternatives exist to accomplish this etch; the preferred methodis to use CHF₃ as the reactive gas at a pressure of 20 mTorr. At thisstage, the exposed surface of the semi-processed device is flat.

Second, a selective etch may be performed on the surface, thatpreferentially etches the cladding layers 742 and 744 compared to thecore 740. The preferred way to perform this etch is to perform achemical etch with a buffered HF solution (BHF: a mixture of ammoniumfluoride and hydrofluoric acid). As is known in the art, BHF etchessilica rapidly, but does not etch GeO₂ at all, so that the GeO₂-richcore layer 740 etches more slowly than the pure silica cladding layers.The exact profile of the protrusion created by this process follows theconcentration profile of Ge. Other means of selective etching may alsobe used, including dry etching. The distance by which the center of thecore projects from the surface 732 depends on the etching parameters(materials, densities, time, temperature, etc.). If there is no uppercladding used for the waveguide 740, i.e. layer 742 is absent (notshown), the shape of the resultant lenticular structure will bedifferent, and asymmetric vertically. As was the case for FIG. 3, adiode laser chip may be aligned and attached to the substrate 770(preferably by flip-chip bonding) so that the axis of the laserwaveguide 320 or 112 is coaxial with the axis 754 of the waveguide 740.

Although for simplicity FIG. 7 shows the core shape being unchanged bythe etching process, in reality, the shape of the protrusion 753 in theregion of the core 740 at the surface 732 has no sharp corners and canbe described by smooth curves as in FIG. 7B, where surface grid linesalong the x-z planes and the y-z planes are shown to give an impressionof the smoothly varying surface shape. FIG. 7B shows the protrusion 753forming a lenticular structure with two different curvatures in the x-zand in the y-z planes, since the height (y-dimension) and width(x-dimension) of the core 740 at the surface 732 are different.Different curvatures are desired because the divergences of the diodelaser mode are quite different in the two planes due to the differenttypical mode sizes, as described above. The small vertical mode sizeleads to strong vertical divergence, so a strong curvature is desired inthe y-z plane. Only a weak (or no) horizontal curvature is needed tocompensate the horizontal beam divergence. To accommodate thisdifference, the approximately square cross-section waveguide 740 may bewidened as shown in the regions 750 for FIG. 7A and 751 for FIG. 7B. Thecurvature in the y-z plane may be adjusted through the selective etchingconditions, with generally larger etching time producing a largercurvature, all other things being equal. The curvature is preferablyadjusted until the vertical divergence of the diode laser beam may becompensated. The curvatures of the protrusion 753 will vary inversely asthe widths of the waveguide in the surface 732, so the desired ratio ofcurvatures (vertical to horizontal) can be obtained by adjusting theratio of the widths (vertical to horizontal). Since a hyperbolic lenssurface has no spherical aberration, the fabrication conditions arepreferably adjusted to obtain a near-hyperbolic profile for theprotrusion 753 in the region near the axis 754 where the mode profilecrosses the surface 732.

In the case of a planar waveguide, the region 740 is very wide laterallycompared to its vertical width, it supports a plurality of differentaxes of propagation in the x-z plane, and the lenticular structure willbe translationally invariant along the x axis, providing focusing mainlyin the vertical y-z plane. Note also that the surface 732 (ignoring theprotrusion) is shown as being locally normal to the axis of propagation754 of the mode of the waveguide 740. In this case, by the symmetry ofthe situation, the local surface of the protrusion at the axis 754 isnormal to that axis. However, this surface may be inclined at an angleto deviate the beam, or curved to provide lateral focusing, or take onanother shape for a different purpose.

In FIG. 7A, the rectangular portion 750 (or 751 of FIG. 7B) of thewaveguide is preferably kept smaller in length (along the direction ofpropagation) than the lateral Rayleigh range so that the optical modedoes not have a significant opportunity to expand in the horizontaldimension between the surface 732 and the beginning of the roughlysquare region 760 of the waveguide 740. If this condition is obeyed, notaper may be needed between the two sections of waveguide 750 and 760.In use of a lensed waveguide coupling section, it may be desirable notto use index matching material. Use of an index matching material hasthe advantage of reducing the Fresnel reflections, but it has theundesired effect of requiring an increased curvature of the surface 732which undesirably increases the fabrication time and tightens themanufacturing tolerances.

In an optional step of the fabrication of the lensed surface 753, theregion may be heated to a temperature near the softening temperature ofthe materials 740, 744, and 742. Above the softening temperature butbelow the melting point, the surface tension of the silica can changethe surface profile. This can be called thermal slumping of the surface.By applying a controlled thermal ramp to the wafer, or to an individualpart, the temperature may be raised above the softening temperature fora time sufficient to allow a reduction in the curvature of theprotrusion to a desired value. This step is of interest if the spatialprofile of the protrusion produced by the selective etch step describedabove is too sharp (as shown in FIG. 7A). Another desirable effect ofthermal slumping is the smoothing of the surface, reducing opticalscatter. The heating may be accomplished of a single chip, the entirewafer, or of individually selected regions. While an oven can be usedfor wafer-scale processing, a laser can advantageously be used to heatthe region around the protrusion 752 or 753 if it is desired to slumpindividual regions. Preferably, a CO₂ laser may be used to provide anenergetic pulse of 10 micron optical radiation that is directed onto thesurface 732 and is partly absorbed in a volume near the surface. If anoptical pulse is applied so that between about 0.5 to 1 J/cm² isabsorbed within the top few microns of the predominantly silicawaveguide material during a period of a few microseconds, thermalslumping will be observed. For longer pulses, more energy will berequired, but the thermal diffusion depth varies approximately as thesquare root of the time, so the required energy increases as the squareroot of the pulse length above a pulse duration of a few microseconds.By controlling the laser pulse length and energy, (and wavelength), andthe number of pulses, the desired degree of slumping can be controlledto approach the desired curvature.

The protrusion 753 may be used to refocus optical radiation. An opticalbeam is propagated along the waveguide 740 towards the protrusion 753.Provided that the material outside the waveguide and across theinterface 732 has a lower index of refraction than the core 740, upontraversing the interface 732, the beam is focussed by the curvature ofthe interface 732, and acquires a converging characteristic. The higherindex central portion of the protrusion retards the phase fronts of themode as it traverses the interface, causing phase front curvaturerelated to the curvature of the protrusion, and focussing the mode. Asthe beam continues to propagate towards a longitudinal position ofminimum beam size, at least one beam dimension continues to shrink orfocus. Another waveguide may be aligned in proximity to this position sothat the refocussed beam can enter the second waveguide with goodcoupling efficiency. If the second waveguide is the active waveguide ofa semiconductor laser, the arrangement described may be a part of aninjection locking apparatus, an external cavity resonator apparatus, anamplifier apparatus, or other structures.

Alternatively, the protrusion 753 may be used in effectively coupling asecond waveguide such as in a semiconductor laser to a waveguide 740. Inthis case, the optical beam is originated inside the diode laser,propagates to the protrusion where it is refocussed from a divergingbeam, and propagates along the axis 754 of the waveguide 740. Again theend of the second waveguide may be aligned relative to the interface 732such that the emission facet is collocated with the minimum focus,within a tolerance to achieve the desired coupling efficiency. If thesecond waveguide is fabricated on the same substrate as the waveguide740, the alignment of the end of the second waveguide is accomplishedlithographically. If the second waveguide is a semiconductor diodelaser, the alignment of the second waveguide end is accomplished duringan attach process between the substrate of the diode laser and thesubstrate 770.

In a further alternative preferred embodiment, an indentation may befabricated instead of a protrusion 753. To fabricate the indentation, aselective etch process may be performed that preferentially etches thecore layer 740 compared to the cladding layers 742 and 744. Thepreferred way to perform this etch is to perform a chemical etch with anaqueous solution of H₂SO₄. Since the etching rate of SiO₂ in thisetchant is nil, while the etching rate of GeO₂ is medium, a processtemperature above room temperature is preferred, such as 30° C. or 50°C. The GeO₂-rich core layer 740 etches more rapidly than the pure silicacladding layers. Again, the profile of the indentation created by thisprocess follows the concentration profile of Ge. Now, with air in theremoval region, the indentation defocusses the mode passing through theinterface, which may be useful for some applications. With an indexmatching fluid in the removal region that has a higher index ofrefraction than the waveguide core 740, focusing is again obtained atthe indentation. The general description of the structures incorporatingthe protrusion and the usage of the protrusion also apply to theindentation provided that attention is paid to the reversal of thefocusing properties according to the index of refraction of the removalregion.

What is claimed is:
 1. A thermally tuned laser source with an athermalresonator comprising: a laser gain medium, an intracavity waveguidesegment optically coupled to said gain medium, and thermo-optic feedbackmeans for defining a resonant laser cavity including said gain mediumand intracavity waveguide segment, said thermo-optic feedback meansincluding heating means for tuning a frequency of operation of saidlaser cavity, wherein said gain medium is characterized by a positiverefractive index change with increase in temperature and saidintracavity waveguide segment is characterized by a negative refractiveindex change with increases in temperature, said gain medium andintracavity waveguide segments having optical path lengths chosen suchthat a round trip optical path in said cavity has a substantially netzero optical length change with increase of temperature profile of saidoptical path.
 2. The source of claim 1 wherein said gain medium is asemiconductor laser medium.
 3. The source of claim 1 wherein said gainmedium further includes an antireflection coating.
 4. The source ofclaim 1 wherein said intracavity waveguide segment comprises a polymerstructure, said polymer material having said negative refractive indexchange for increases in temperature.
 5. The source of claim 1 whereinsaid intracavity waveguide segment is butt coupled to said gain medium.6. The source of claim 1 wherein said thermo-optic feedback meansincludes a grating structure.
 7. The source of claim 6 wherein saidgrating structure is formed on a second waveguide segment coupled tosaid optical path in said cavity, said second waveguide segment having atemperature dependent refractive index, and wherein said heating meanschanges a temperature of said second waveguide segment.
 8. The source ofclaim 1 wherein said resonant laser cavity is a bidirectional resonator.9. The source of claim 1 wherein said optical path in said cavity has alength that defines a free spectral range of said resonant cavity thatis a rational fraction of a specified communication frequency channelspacing within an optical frequency band corresponding to said gainmedium.
 10. The source of claim 1 wherein said optical path in saidcavity has a length that defines longitudinal modes of said resonantcavity and said tuning comprises switching said frequency of operationbetween said longitudinal modes.
 11. The source of claim 1 wherein saidresonant laser cavity is a ring resonator.
 12. The source of claim 1wherein said increase in said temperature profile is approximatelyuniform across said gain medium and intracavity waveguide segment. 13.The source of claim 1 wherein said increase of temperature profileresults from environmental temperature change.
 14. The source of claim 1wherein said increase of temperature profile occurs with constantexcitation of said gain medium.
 15. The source of claim 1 wherein saidheating means overlaps a portion of said optical path and wherein saidincrease of temperature profile occurs with constant excitation of saidheating means.
 16. The source of claim 1 wherein in another portion ofits structure said intracavity waveguide segment is characterized by apositive refractive index change with increase in temperature.
 17. Atunable laser source comprising: a laser gain medium, an intracavitywaveguide segment optically coupled to said gain medium, andelectro-optic feedback means for defining a resonant laser cavityincluding said gain medium and intracavity waveguide segment, saidelectro-optic feedback means including electrode means for tuning afrequency of operation of said laser cavity, wherein said gain medium ischaracterized by a positive refractive index change with increase intemperature and said intracavity waveguide segment is characterized by anegative refractive index change with increases in temperature, saidgain medium and intracavity waveguide segments having optical pathlengths chosen such that a round trip optical path in said resonantcavity has an optical length that is substantially independent ofambient temperature over a specified ambient temperature range.
 18. Thesource of claim 17 wherein said gain medium is a semiconductor lasermedium.
 19. The source of claim 17 wherein said gain medium furtherincludes an antireflection coating.
 20. The source of claim 17 whereinsaid intracavity waveguide segment comprises a polymer structure, saidpolymer material having said negative refractive index change forincreases in temperature.
 21. The source of claim 17 wherein saidintracavity waveguide segment is butt coupled to said gain medium. 22.The source of claim 17 wherein said electro-optic feedback meansincludes a grating structure.
 23. The source of claim 22 wherein saidgrating structure is formed on a second waveguide segment coupled tosaid optical path in said cavity, said second waveguide segment havingan electric field dependent refractive index, and wherein said electrodemeans changes the electric field in said second waveguide segment. 24.The source of claim 23 wherein said second waveguide segment comprises apoled polymer structure, said poled polymer structure having saidelectric field dependent refractive index.
 25. The source of claim 17wherein said resonant laser cavity is a bidirectional resonator.
 26. Thesource of claim 17 wherein said optical path in said cavity has a lengththat defines a free spectral range of said resonant cavity that is arational fraction of a specified communication frequency channel spacingwithin an optical frequency band corresponding to said gain medium. 27.The source of claim 17 wherein said optical path in said cavity has alength that defines longitudinal modes of said resonant cavity and saidtuning comprises switching said frequency of operation between saidlongitudinal modes.
 28. The source of claim 17 wherein said resonantlaser cavity is a ring resonator.
 29. The source of claim 28 whereinsaid ring resonator contains a grating assisted coupler.
 30. The sourceof claim 17 wherein in another portion of its structure said intracavitywaveguide segment is characterized by a positive refractive index changewith increase in temperature.
 31. The source of claim 17 wherein saidelectro-optic feedback means contains a liquid crystal material, saidliquid crystal material having said electric field dependent refractiveindex.
 32. The source of claim 31 wherein said liquid crystal materialis a polymer dispersed liquid crystal.
 33. An athermal laser sourcecomprising: a laser gain medium, an intracavity waveguide segmentoptically coupled to said gain medium, and frequency selective feedbackmeans having a feedback bandwidth and defining a resonant laser cavityincluding said gain medium and intracavity waveguide segment, saidfeedback bandwidth being sufficiently narrow compared to the freespectral range of said cavity so that only one of the longitudinal modesof said cavity is substantially excited by said gain medium, whereinsaid gain medium is characterized by a positive refractive index changewith increase in temperature and said intracavity waveguide segment ischaracterized by a negative refractive index change with increases intemperature, said gain medium and intracavity waveguide segments havingoptical path lengths chosen such that a round trip optical path in saidresonant cavity has an optical length that is substantially independentof ambient temperature over a specified ambient temperature range. 34.The source of claim 33 wherein said gain medium is a semiconductor lasermedium.
 35. The source of claim 33 wherein said gain medium furtherincludes an antireflection coating.
 36. The source of claim 33 whereinsaid intracavity waveguide segment comprises a polymer structure, saidpolymer material having said negative refractive index change forincreases in temperature.
 37. The source of claim 33 wherein saidintracavity waveguide segment is butt coupled to said gain medium. 38.The source of claim 33 wherein said frequency selective feedback meansincludes a grating structure.
 39. The source of claim 38 furthercomprising a heating means disposed in said frequency selective feedbackmeans, for tuning a frequency of operation of said laser cavity whereinsaid grating structure is formed on a second waveguide segment coupledto said optical path in said cavity, said second waveguide segmenthaving a temperature dependent refractive index, and wherein saidheating means changes the temperature in said second waveguide segment.40. The source of claim 39 wherein said second waveguide segmentcomprises a polymer structure, said polymer structure having saidtemperature dependent refractive index.
 41. The source of claim 33wherein said resonant laser cavity is a bidirectional resonator.
 42. Thesource of claim 41 wherein said bidirectional resonator contains agrating assisted coupler.
 43. The source of claim 33 wherein saidoptical path in said cavity has a length that defines a free spectralrange of said resonant cavity that is a rational fraction of a specifiedcommunication frequency channel spacing within an optical frequency bandcorresponding to said gain medium.
 44. The source of claim 33 whereinsaid optical path in said cavity has a length that defines longitudinalmodes of said resonant cavity and said tuning comprises switching saidfrequency of operation between said longitudinal modes.
 45. The sourceof claim 33 wherein said resonant laser cavity is a ring resonator. 46.The source of claim 33 wherein in another portion of its structure saidintracavity waveguide segment is characterized by a positive refractiveindex change with increase in temperature.
 47. The source of claim 33wherein said frequency selective feedback means is substantiallyindependent of temperature.
 48. The source of claim 47 wherein saidfrequency selective feedback means comprises an electro-optical feedbackelement, and an electrode actuator for applying an electric field insaid feedback element to produce a change in refractive index of saidfeedback element for tuning said feedback bandwidth.
 49. The source ofclaim 33 wherein said frequency selective feedback means comprises athermo-optical feedback element, and a thermal actuator for heating saidfeedback element to produce a change in refractive index of saidfeedback element for tuning said feedback bandwidth.
 50. A ring laserapparatus comprising: a laser gain medium, first and second waveguidesegments optically coupled to said laser gain medium; and a frequencyselective means for coupling optical energy between said first andsecond waveguide segments and also coupling a portion of said opticalenergy out of at least one of said waveguide segments to form an opticaloutput, wherein said laser gain medium and said optically coupled firstand second waveguide segments form a ring resonator having a round tripoptical path characterized by a round trip optical length, at least oneof said waveguide segments characterized by a negative refractive indexchange with increases in temperature and having a length chosen suchthat the round trip optical length is athermal.
 51. The apparatus ofclaim 50 wherein at least one of said waveguides is coupled to anoptical output.
 52. The apparatus of claim 50 wherein said frequencyselective means comprises a reflective grating.
 53. The apparatus ofclaim 52 wherein said reflective grating is formed over a pair ofwaveguides coupled to said respective first and second waveguidesegments at a first end, a second end of at least one of said waveguidecoupled to an output optical port.
 54. The apparatus of claim 50 whereinsaid frequency selective means comprises a forward coupling grating,having a first waveguide coupled to said first waveguide segment at afirst end of said grating and a second waveguide coupled to said secondwaveguide segment at a second end of said grating, said first and secondwaveguides having different propagation constants and said gratingcharacterized by a wavenumber matched to the difference in propagationconstants between the two waveguides.
 55. The apparatus of claim 54wherein one of said first and second waveguides is characterized by anegative refractive index change with increases in temperature and hasheating means associated therewith for tuning said frequency selectivemeans.
 56. The apparatus of claim 55 wherein the other of said first andsecond waveguides is characterized by a substantially equal and oppositepositive refractive index charge with increases in temperature and alsohas heating means associated therewith.
 57. The apparatus of claim 55wherein said laser gain medium has heating means operated incoordination with said heating means associated with said forwardcoupling grating.
 58. The apparatus of claim 54 wherein one of saidfirst and second waveguides is characterized by an electric fielddependent refractive index and has electrode means associated therewithfor applying an electric field thereto.
 59. The apparatus of claim 50wherein said round trip optical length defines a free spectral range ofsaid ring resonator that is a rational fraction of a specifiedcommunication frequency channel spacing.
 60. An athermal ring lasersource comprising: a laser gain medium, an first intracavity waveguidesegment optically coupled to said gain medium, an second intracavitywaveguide segment optically coupled to said gain medium, and frequencyselective feedback means disposed to couple optical energy between saidfirst and second waveguide segments having a feedback bandwidth anddefining a resonant ring cavity including said gain medium and saidfirst and second intracavity waveguide segments, wherein said gainmedium is characterized by a positive refractive index change withincrease in temperature and said first intracavity waveguide segment ischaracterized by a negative refractive index change with increases intemperature, said gain medium and first intracavity waveguide segmentshaving optical path lengths chosen such that a round trip optical pathin said resonant ring cavity has an optical length that is substantiallyindependent of ambient temperature over a specified ambient temperaturerange.
 61. The source of claim 60 wherein said frequency selectivefeedback means is a grating structure.
 62. The source of claim 61wherein said grating structure is a reflective grating.
 63. The sourceof claim 60 wherein optical energy is output from both said first andsecond intracavity waveguide segments.